Preheat your oven to 400°F. Line a large, rimmed baking sheet with parchment or foil.
Cut the mini peppers in half lengthwise. I usually leave the stem, as I think it's pretty, but you can cut it off if you wish. The larger mini peppers might have seeds and membranes - remove those with your fingers or a paring knife.
Place the peppers on the prepared baking sheet. Toss them with olive oil, salt, pepper, garlic powder, oregano, and parmesan, and arrange them in a single layer on the pan.
Bake the peppers until tender, for about 20 minutes.
Serve immediately.
