Padrón peppers
Small green peppers from Galicia, usually mild, occasionally surprisingly hot.
In summer they turn up in many supermarkets and at farmers' markets, otherwise in Spanish delis or from produce delivery services. They are usually fried whole in olive oil until the skin blisters and eaten with coarse salt.
Substitute:
Shishito peppers from an Asian grocer are almost identical, just slightly thinner-walled and slimmer. Italian friggitelli also work but are sweeter and almost never hot.