How to Slice a Tomato
To slice a tomato, first wash it under cool water and place it on your cutting board. Core the tomato by poking the tip of a paring knife into the base of the stem. Move the blade around the green stem at an angle to remove the top 1 inch (2.5 cm) of the tomato. Pull the core out by hand. Then, set the tomato on its side. Hold the blade of a serrated knife on top of the tomato and line your first cut up next to the top of the tomato. Slide the blade back and forth while bracing the rest of the tomato to cut your first slice. Repeat this process by cutting the tomato into slices that are roughly the same thickness. Make each cut parallel to your previous cut to avoid making misshaped slices.
Place your washed tomato on a cutting board. Make sure that the stem end is facing upwards. This will make coring the tomato easier. Remove the core. If you're working with large roma or beefsteak tomatoes, you'll have to remove the core before slicing them. If you're working with small cherry or grape tomatoes, this step is unnecessary and you can skip ahead to the next step. Place your thumb right below the edge of a knife, preferable a small paring knife. Use your non-cutting hand to grab the base of the tomato, and stabilize your hand against the cutting board. Place your knife's tip slightly to the side of the core. Push your knife into the tomato about ⁄ 4 – ⁄ 2 inch (0.64–1.27 cm) deep. Cut out a small circle by twisting your knife. Pull up to remove the core. Place the tomato on its head. Now that the core is removed, your tomato will have a flatter top. Turn your tomato upside down so that it faces the cutting board and gives you a stable base. Cut the tomato in half. Using a sharp chef's knife or a serrated knife, make 1 long cut from top to the bottom so that your tomato is divided into 2 even hemispheres. This will make slicing much easier, but if you want whole tomato slices, you can omit this step and skip ahead to the slicing part. Take 1 of the halves and reposition it. Place the cut side down, facing the cutting board. Slice the tomato. The following steps will help you make safe, even cuts across your tomato. If you are slicing a whole tomato, you can still follow the same steps. Place your non-cutting hand on the left side of the tomato (if you are cutting with your right hand). Curl up your fingers into a claw shape and press your fingers tips lightly on the tomato. This type of grip helps to stabilize the tomato and prevent you from cutting yourself. Place the tip of the knife on the cutting board behind the rightmost side of the tomato. Keeping the knife tip on the cutting board, drag your knife straight down and through the tomato. A sharp knife will make cutting easier. Once you are through to the other side, lift up the knife. Reposition your knife at the top of the tomato, about ⁄ 4 – ⁄ 2 inch (0.64–1.27 cm) to the left of your last cut, depending on how thick you want your slices. Repeat the same dragging motion and continue across the tomato. Repeat for the second tomato half. Enjoy! Now you have a perfectly sliced tomato for salads, sandwiches, or just as-is. Drizzle your slices in olive oil or spice them up with a little salt, pepper, or cumin to bring out their natural flavor. Set up your mandoline. Mandolines are a staple cutting tool for professional chefs and at-home meal makers alike and are handy for making lots of uniform cuts quickly. Follow the manufacture's setup instructions so that your mandoline is standing upright on your cutting board. Set the blade. This will determine the thickness of your tomato slices. Depending on the make and model of your mandoline, you can dial-in or adjust your blade's distance from the mandoline's body to make thicker or thinner slices. Make sure your blades are sharp—this will help you make really thin slices without seizing up the machine or ripping the tomato. Load your tomato. Mandolines almost always come with a hand-guard to hold the object you're planning to cut. Some models have spikes inside the holder to help secure slippery items like tomatoes. Mandoline blades can be very sharp and can slice neatly into exposed fingertips, so always use the hand-guard to protect yourself! Cut your tomato. The mandoline is going to do most of this for you. Start at the top of the mandoline body and slide the tomato and hand-guard down and over the blades. Try using a slight zig-zag motion to move over the blades; this will help to cut through the tomato's skin and not squish it. Return to the top and drag down again to create another slice. Keep repeating this step until you reach the end of your tomato. Enjoy! Your slices will all be same thickness, which is especially great for even cooking times and ensuring everything is cooked. Get out your tomato slicer. One popular hand-operated model looks like a bit like a flyswatter. Put your clean tomato on a cutting board. Use your non-dominant hand to keep it stable and in place. Cut your tomato. Your slicer will make quick work of your tomato, slicing the whole thing all at once! Hold the tomato slicer handle in your dominant cutting hand, with the sharp end of the blades facing the tomato. For extra leverage and stability, place your pointer finger on top of the handle where it intersects with the slicer's frame. Use a sawing motion to work the slicer's blades through the tomato. When you get about halfway through the tomato, move your non-cutting stabilizing hand to the top of the tomato so that it's not in the way of the blades. You don't want to cut yourself! Keep sawing back and forth through the tomato until you reach the cutting board. Finish. Once your reach the cutting board, you should have a perfectly sliced tomato. Pull out your egg or apple slicer. This is a fun alternative way to cut tomatoes. It's particularly helpful if you're squeamish with blades and don't feel comfortable with your knife skills, or if you're just looking for another way to repurpose that egg or apple slicer taking up space in your drawer. Egg slicers can work great for smaller tomatoes, while an apple slicer will work better on beefsteak tomatoes. Load your washed tomato. Simply place your tomato into the slicer's center cradle, as you would a hardboiled egg or an apple. Cut your tomato. Gently yet quickly press down on the cutting lever to slice through your tomato. Finished. Enjoy! 
Place your washed tomato on a cutting board. Remove the core. Place the tomato on its head. Cut the tomato in half. Take 1 of the halves and reposition it. Slice the tomato. Enjoy! Set up your mandoline. Set the blade. Load your tomato. Cut your tomato. Enjoy! Get out your tomato slicer. Put your clean tomato on a cutting board. Cut your tomato. Finish. Pull out your egg or apple slicer. Load your washed tomato. Cut your tomato. Finished. 