In a large bowl, toss onion, red bell pepper, and green bell pepper with olive oil, salt, and pepper.
GRILL:
Grill vegetables until they are softened and slightly charred, about 5-7 minutes. **see notes
Remove the veggies from the grill and set aside.
Grill bratwursts until they are cooked through and have grill marks on all sides, about 10-12 minutes. Turn the bratwursts every few minutes to ensure even cooking.
Once the bratwursts are fully cooked, remove them from the grill and rest for a few minutes.
TOAST BUNS:
Toast the buns on the grill until they are slightly charred and crispy.
TO FINISH:
To assemble the brats, place a bratwurst in each bun, then top with the grilled vegetables and any other desired toppings.
Notes
**You can also use a grill pan to cook the vegetables.