Preheat oven to 400 degrees F. Slice bell peppers and onion into strips. Place these into a bowl.
Toss veggies with olive oil and salt and pepper leaving a little oil on the side to drizzle on brats.
Cover baking sheet with foil. Pour veggies on top in a single layer. Lay bratwurst on top of them. Drizzle remaining oil on meat.
Cook for 25-30 minutes turning meat half way thru cook time. Once middle of meat has reached 160 degrees they are safe to eat. If you want to crisp outsides, broil for 2 minutes or so and then serve on buns with pepper mix and mustard.