1Preheat your grill to medium-high heat (about 400°F).
2Cut peppers in half lengthwise, remove the seeds, and cut into thick strips. Slice the onion into thick wedges, keeping them attached at the root.
3Toss peppers and onions with 1 tbsp oil, salt, pepper, and garlic powder in a large skillet.
4Place bratwurst directly on the grill grates and cook for 12–15 minutes, turning every few minutes, until deep golden and cooked through. Let rest 2 minutes.
5While brats cook, heat the skillet on a burner to medium-high and sauté the pepper and onion mixture for 8–10 minutes, stirring occasionally, until soft and lightly charred at the edges. Add thyme if using in the last minute.
6Serve brats on toasted rolls with the pepper-onion mixture piled on top.