In this recipe, what is a possible substitute for serrano pepper?

Question

In this recipe, I enjoy the mix of spices: serrano, cilantro, mint, cinnamon...

Next time we'll cook it, we won't have serrano available. What could be a substitute for it?

Ideally this substitute is not hard to get in Europe...

Note the serrano's contribution to the dish. They're not opened. They're not eaten. They simmer with the rest of the ingredients perhaps adding some taste.

EDIT: Thanks for the suggestions. I now realize that substitute may not have been the best wording. Given the constraints, a simple s/serrano/xxxx/ replacement may not do ☺

Answer

You want something hot and very slightly herby, without smoke. For heat, a moderate amount of cayenne is the easiest option. For the herby notes, I might go with something like marjoram or rosemary, but frankly given the cilantro and mint I probably wouldn't bother.

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