A spicy vegetable which stays in a sandwich? I make a vegetable sandwich which I find very good. It has tomato, red onion, avocado, and cucumber. However, it lacks spiciness. I want to avoid spreads and use a vegetable for mild heat.

I tried an anaheim pepper. It has some kick but the texture does not work. It slides out. With the cucumber I cut them the length of the sandwich so they have structure.   

Is there something with texture of cucumber but the spice of mild pepper?
My home pickled beans get spicy because there's a dried red pepper or two in the jar, along with a dill head and some garlic cloves. The beans are very nice in a sandwich. But you could pickle-and-spice anything at all and use it to jazz up the sandwich. The texture is that of a soft vegetable.

Also, look into mustard greens and their relatives. They look like lettuce or kale, but taste distinctly spicy. The texture is more lettuce-y than cucumber or slices of pepper, but it wouldn't be out of place on a sandwich.