Preheat oven to 375ºF with a rack in the middle position.

Prick the sausages several times all over with a fork, place on a rimmed baking sheet, and roast, turning once, until nicely browned. Transfer to a platter and scrape pan drippings (you’ll use the same baking sheet to roast the vegetables) into a small saucepan; add enough olive oil to make about ¼ cup. Place over medium heat and add tomato paste; cook, stirring, until the paste begins to dissolve in the oil and is quite fragrant, 2 to 3 minutes.

Toss onion, peppers, and garlic in a large mixing bowl; drizzle with olive oil/tomato paste mixture and sprinkle generously with Italian seasoning and salt. Toss to coat and spread evenly on baking sheet.

Roast, stirring occasionally, until beginning to brown, 15 to 20 minutes; add the sausages and roast until sausages are hot and vegetables are browned around the edges, about 5 minutes more.

Toast sandwich rolls if desired; spread with provolone and/or mustard, if using. Add a sausage to each roll and top with roasted vegetables; drizzle each with some of the pan drippings. Serve at once with plenty of napkins.