Cook the Italian sausage in a skillet or bake until fully cooked; drain any fat, let cool slightly, then halve lengthwise and slice into 1/2-inch pieces.
Heat 2 tablespoons olive oil in a skillet over medium heat and sauté the chopped green bell pepper until it is no longer crisp but not fully soft, about 4–6 minutes; set aside.
In a heavy saucepan, combine the canned tomatoes (crush by hand if desired for a chunky texture), tomato puree, minced garlic, 4 tablespoons olive oil, and fresh basil; bring to a light simmer over medium heat.
Add the sautéed peppers and sliced cooked sausage to the sauce and simmer together for 3–5 minutes; season with salt and pepper to taste.
Cook the ziti according to package directions until al dente, reserving 1/2 cup pasta cooking water if needed, then drain.
Toss the cooked ziti with the marinara sauce, adding a splash of reserved pasta water if the sauce needs loosening, then serve topped with the sausage-and-pepper mixture.
Garnish each serving with chopped parsley and grated Parmesan cheese before serving.