Fire up the grill for a two-zone setup. If you are using charcoal, pile your lit coals on one side of the grill to create a hot direct zone hitting around 400 to 425 degrees Fahrenheit, leaving the other side empty for indirect heat. If you are on a gas grill, crank the burners on one side to high and leave the other side off. Let the grill preheat with the lid closed for 10 to 15 minutes so the grates get fully up to temp. Clean and oil those grates before anything goes on - a clean grate is the foundation of a clean sear.

While the grill is preheating, prep your vegetables. Slice the bell peppers into thick half-inch strips and cut the onions into half-inch rings. Toss them in a bowl with the olive oil, kosher salt, black pepper, garlic powder, and Italian seasoning until everything is well coated. Set them aside. If you are using a grill basket, load the peppers and onions in. If you are going direct on the grates, make sure the strips are thick enough not to fall through.

Place your Italian sausage links directly over the hot zone. You want to hear that immediate sizzle when they hit the grates - that tells you the grill is ready. Do not pierce the casings at any point. Sear each side for about 2 to 3 minutes, rotating the links a quarter turn at a time to build even char marks on all sides. Work around all four sides of each link before moving to indirect heat. This initial sear locks in those juices and gives you that snappy, caramelized casing that makes the sandwich.

Once all sides are seared with good color, move the sausages to the indirect heat side of the grill. Close the lid and let them finish cooking low and slow on indirect heat for another 10 to 12 minutes. This is where the center comes up to temp without the outside burning. You are looking for an internal temp of 160 degrees Fahrenheit measured at the thickest part of the link. Pork sausage does not have a smoke ring or bark the way a long brisket cook does, but the casing should be deep mahogany brown and slightly firm to the touch.

While the sausages finish on indirect heat, get your peppers and onions on the hot zone. Place the grill basket or lay strips directly on the grates over the direct heat side. Cook them for 8 to 10 minutes, tossing or flipping every couple of minutes, until they are softened, lightly charred at the edges, and sweet smelling. You want some char on those peppers - it adds a smoky depth that ties the whole sandwich together. Season with a little extra salt and pepper once they come off the grill.

Pull the sausages off the grill once they hit 160 degrees Fahrenheit internal temp and let them rest for 3 to 5 minutes on a cutting board before building the sandwiches. Always rest the meat - even sausages benefit from a brief rest to let the juices redistribute throughout the link. While the sausages rest, brush the cut sides of your hoagie rolls with the softened butter and lay them cut-side down directly on the hot grates for 60 to 90 seconds until they are golden and toasted. Watch them closely because buttered bread can go from toasted to torched in seconds.

Build your sandwiches while everything is still hot. Spread a generous layer of whole grain mustard on the toasted cut sides of each roll. Lay a sausage link in the roll, pile on a heap of the grilled peppers and onions, and if you are going with provolone, lay a slice over the top and close the lid for 30 to 60 seconds to let the residual heat melt the cheese right on the grill. Serve immediately and have plenty of napkins on hand because this sandwich is gloriously messy in all the right ways.