Preheat the oven to 400ºF with the rack in the center position
In a small bowl, add olive oil, red wine vinegar, mustard, garlic powder, salt and pepper. Mix till evenly combined. Set this vinaigrette aside
Slice onion and bell peppers to 1/4 inch strips
Add them to a disposable baking pan
Add the raw bratwursts
Stir the vinaigrette once and drizzle into the pan. Toss the bratwurst, peppers and onions till the vinaigrette has evenly coated everything
Place the baking pan in the oven and roast for 15 minutes.
After 15 minutes, toss everything again and make sure the bratwursts are flipped
Bake again for 20 minutes till the bratwursts, onions and peppers have browned edges. Check the internal temperature of the bratwursts. It should be 160 F to be fully cooked
You can serve as is, or you can stuff them into hot-dog buns along with the veggies. Spread some cheddar on top and put them back in the oven for about 5 minutes for the buns to be toasted and the cheese to melt
Add mustard sauce or any other toppings as needed.