Put pepper and onion slices on a baking sheet, drizzle with the olive oil, add any seasonings you like to use, and toss everything with your hands. This can also be done in a bowl with a spoon if you prefer.
Spread the vegetables out on the baking sheet and bake for about 20 minutes, until the peppers and onions are soft and lightly brown.
Pan-Fried:
Heat a large skillet over medium-high heat.
Add in olive oil, bell peppers and onion strips.
Add salt and pepper to taste and any other seasonings you want to use.
Cook, stirring occasionally for about 10-15 minutes until vegetables have softened and start to caramelize.
Notes
For use in fajitas, tacos, or burritos add 1 tsp. each of cumin, chili powder and oregano.