A spicy vegetable which stays in a sandwich?

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.

For this specific requirement I eat sliced cherry peppers in my sandwiches. While they are not the length that you could cut cucumbers, I have little problem keeping them in the sandwich. I have about as much trouble keeping tomatoes in the sandwich, and I rarely have to replace them.

Deseeding the pepper thoroughly and slicing it into millimeter thick rings should make it stay put. You could sprinkle the rings on like a green herb.

Watercress, sorrel, nasturtium leaves (or Nasturtium flowers, but the leaves will work better in a sandwich.) All are spicy, but it's not "hot pepper flavor."

You could pickle some peppery (spiced up) cucumber slices - slice them and marinate with hot peppers in the refrigerator (assuming you don't want to go to the bother of canning them, "refrigerator pickles" are an easier approach.)

Non-prefabricated pepper spread could be as easy as mashing a pepper, perhaps with a little oil, to get a "spreadable" consistency.

One way to make your sandwich spicy would be kimchi, or if you have the time, you can make your own spicy pickled cabbage in your refrigerator, as explained in this recipe for hasperat.

You can try some thinly sliced horseradish. It'll have a nice kick and slight crunch.