01 - Set your outdoor grill or stovetop grill pan to medium-high heat and allow it to reach full temperature before cooking.
02 - Brush the tomato halves, onion rings, bell pepper quarters, and garlic cloves generously with olive oil on all sides to ensure even charring.
03 - Place the Italian sausages on the hot grill and cook, turning occasionally with tongs, until cooked through with a nicely charred exterior, 12 to 15 minutes.
04 - While the sausages cook, grill the tomatoes cut side down along with the onion, bell pepper, and garlic until charred and softened, 5 to 7 minutes per side.
05 - Transfer the grilled tomatoes, onion, bell pepper, and garlic to a food processor or blender. Add the dried oregano, dried basil, and crushed red pepper flakes. Season with salt and black pepper to taste, then pulse until a chunky rustic sauce forms.
06 - Split the hoagie rolls open and place them face down on the grill. Toast for 1 to 2 minutes until the cut surfaces are golden and lightly crisp.
07 - Arrange a grilled sausage in each toasted roll. Spoon a generous amount of the grilled tomato sauce over each sausage and sprinkle with shredded mozzarella and grated Parmesan if desired.
08 - Return the assembled grinders to the grill, close the lid, and cook for 2 to 3 minutes until the cheeses are fully melted and bubbly.
09 - Remove from the grill, garnish with fresh basil leaves, and serve immediately while hot.