In this recipe, what is a possible substitute for serrano pepper? 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 ☺
The closest thing that's at all common in grocery stores in my experience is just a jalapeño. There might be some alternatives, but I think they'd be even harder to find than serranos. (This is based on my experience in the US, but I'm guessing it'll be true in Europe too.)

Failing that, just use green bell pepper. The flavor is similar, minus the heat, and you can make up the heat with whatever you want, like cayenne or another mostly-just-hot ground chili.