Trim and thinly slice 3 large bell peppers (about 5 cups). Thinly slice 1 medium yellow onion (about 1 ½ cups). Thinly slice 3 garlic cloves.

Heat 1 tablespoon of the olive oil in a large skillet over medium heat until shimmering. Add 1 pound Italian pork sausage links in a single layer. Cook, turning halfway through, until browned on both sides, 7 to 10 minutes total. Transfer to a cutting board or plate. The sausage will not be cooked through.

Add the remaining 1 tablespoon olive oil to the pan and heat until shimmering. Add the peppers, onion, ¾ teaspoon kosher salt, and ¼ teaspoon black pepper. Cook, stirring occasionally, until the onions are translucent and the peppers are slightly softened, 7 to 10 minutes. Meanwhile, cut the sausage crosswise on a slight diagonal into ½-inch pieces.

Add the garlic, 2 tablespoons tomato paste, and 1 teaspoon Italian seasoning to the pan and stir to coat. Cook, stirring occasionally, until the tomato paste is darkened in color, about 2 minutes. Add 1 (14.5-ounce) can diced tomatoes and ½ cup water. Scrape up any browned bits from the bottom of the skillet. Return the sausage and any accumulated juices to the pan, and bring to a simmer.

Reduce the heat as needed to maintain a gentle simmer and cook, stirring occasionally, until the flavors meld and the sauce is slightly thickened, 15 to 20 minutes. Garnish with chopped fresh basil leaves if desired.