In a large mixing bowl, add sliced pepper and onions.
In a small bowl, add the veggie drizzle ingredients and whisk to combine.
Pour the veggie drizzle over the top of the pepper and onions, then toss to combine.
Spray a large, rimmed sheet pan or casserole dish with cooking spray. For easy cleanup, you can also line it with aluminum foil.
Then, place the uncooked brats on one side of the baking sheet.
On the other side of the baking sheet, place the peppers and onions.
Roast in the preheated oven for 10 minutes, then remove to flip the bratwurst.
Roast for another 8-10 minutes at 400 degrees Fahrenheit. Remove when the brats reach an internal temperature of 160 degrees Fahrenheit using an instant read meat thermometer.
(Optional): Remove cooked peppers and onions, then broil the brats on high for 1-2 minutes to help the brats reach a nice golden brown.
Allow the brats to rest for 2-3 minutes before serving. Place on a bun and top with the mustard of your choice and roasted peppers and onions.