Preheat your oven to 400 degrees F. Line a large rimmed baking sheet with 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 some seeds and membranes - remove those with your fingers or with a paring knife.
Place the peppers in a large bowl. Toss them with the olive oil, salt, pepper, garlic powder, oregano, and Parmesan.
Arrange the seasoned mini peppers in a single layer on the prepared baking sheet.
Bake them until tender, about 20 minutes. Serve immediately.
Instead of roasting, you can broil the mini peppers. Broil them in a rimmed broiler-safe pan, directly below the heating element, until tender and just beginning to char, about 2 minutes per side. Keep a close eye on them so that they don't burn.