Preheat the 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.
Video
Notes
If your baking sheet is broiler-safe, you can make this recipe even faster if you broil the peppers instead of roasting them. Broil them on a foil-lined, rimmed baking sheet for about 5 minutes, six inches below the heating element (not directly below). Keep a close eye on them so that they don't burn.
To make this recipe dairy-free, omit the parmesan.
You can keep leftovers in an airtight container in the fridge for up to 4 days. Reheat them in the microwave at 50% power. They are also excellent when served cold, antipasti-style.