Shishito peppers
Small, thin-walled green peppers from Japan, mild and slightly grassy, with the occasional hot one among them.
Shishito peppers are sold in Asian grocers, well-stocked supermarkets and at farmers' markets, but not everywhere. The closely related Korean variety, kkwari-gochu, sometimes turns up fresh in Korean grocery stores.
Substitute:
Padrón peppers come closest, though their skin is slightly thicker and they are more often hot. Young green sweet pointed peppers cut into pieces also work, but they are juicier and less aromatic.