Preheat your grill to medium heat (about 375–400°F). Clean and oil the grates.
Place the brats on the grill and cook for about 12–15 minutes, turning occasionally, until browned and cooked through (internal temp 160°F). If you like extra juicy brats, you can parboil them in beer for 8–10 minutes before grilling—but it’s optional. Toast the buns for 1-2 minutes until golden brown.
If you do not have a grill basket, add the peppers to a large piece of foil. Drizzle with olive oil. Toss. Sprinkle with garlic powder, onion powder, paprika and salt. Toss to combine. Fold the foil over in half. Crimp the edges so you have a pocket. If you have a grill basket, add the peppers to the basket. Drizzle with oil. Toss. Sprinkle with seasoning, toss. Grill for 8-10 minutes, stirring half way through until soft and slightly charred.
While everything is grilling, make the sauce. Melt butter in a medium saucepan over medium heat. Add garlic and cook for 30 seconds, just until fragrant. Whisk in flour and cook for 1–2 minutes to form a roux. Slowly whisk in warm milk, making sure it stays smooth. Bring to a gentle simmer until slightly thickened, about 3–4 minutes. Turn heat to low and stir in the provolone cheese and dijon until the cheese is melted and smooth. Season with salt and pepper. If it gets too thick, add a splash of milk. If too thin, let it simmer another minute.
Place each brat in a toasted bun, pile on the grilled peppers and onions, then spoon over plenty of warm provolone cheese sauce.