How do I prevent tomato making quiche soggy I have made a simple quiche using a Martha Stewart recipe (http://www.marthastewart.com/336904/spinach-and-gruyere-quiches) several times, sometimes altering the filling to include variously bacon, ham, mushrooms, cheddar, and today, tomato slices with basil leaves. 

I typically half the amount of filling and make only one quiche. Usually, this comes out quite well (although sometimes a bit overly fluffy like a souffle) and makes for a quick supper. I use prepared pastry (usually Pillsbury) and it typically comes out well-cooked and not soggy, despite not blind-baking the crust which is apparently something I should be doing based on this question: Why is my quiche soggy?. 

Until today, the only time I had a problem was when I used spinach without pressing out some of the moisture, and then the quiche filling and crust were both soggy. 

Today, I added sliced plum tomato on the top of the quiche before cooking, but found I had to cook the quiche for an additional 10 minutes and still had a somewhat overly gooey filling (but the crust was basically fine).

Unlike spinach, it seems silly to squeeze out tomato slices so that they don't add additional moisture to the quiche. Is there a strategy to avoid this problem?
You may wish to seed the tomatoes, removing the gelatinous part containing the seeds, which is mostly water, and very little flavor.  You want to use only the meaty, fleshy part of the tomato in a quiche.  

Depending on the size of your tomatoes, scooping the seeds out with a melon scoop, cutting out the seed sections, or simply squeezing out the seeds will help.  The Shiksa in the Kitchen has a very nice article with very clear pictures showing these three different methods of seeding tomatoes.

This will substantially reduce the amount of moisture the tomatoes are adding to your quiche.

Another method you may choose to use, if you have the time, is to roast your tomatoes before using them in the quiche (which may also involve seeding them first).  This will reduce the moisture and concentrate the flavor.