Boil pasta water. Bring a large pot of generously salted water to a boil. Keep it ready while you start the sausage.
Brown the sausage. Heat the olive oil in a 12-inch deep skillet over medium-high heat. Add the sausage and break it into rough nickel-sized chunks. Don't stir for the first 3 minutes — let the bottom develop a deep brown crust. Then break it up further and cook another 4-5 minutes until no pink remains and the edges are deeply browned.
Add the bell pepper. Push the sausage to one side of the pan and add the diced bell pepper. Cook in the rendered sausage fat for about 4 minutes, until the pepper is just-tender and starting to char at the edges.
Cook the pasta. Add the pasta to the boiling water and cook 1 minute less than the package directs. Reserve ½ cup of pasta water before draining.
Add the marinara. Pour the full jar of marinara into the skillet with the sausage and peppers. Stir to combine, scraping up any browned bits from the bottom of the pan. Reduce heat to medium-low and simmer 4-5 minutes.
Combine. Add the drained pasta directly to the skillet and toss to coat. Add a splash of reserved pasta water if the sauce looks tight.
Finish with cheese. Remove from heat. Scatter the cheese over the top and cover the skillet for 2 minutes to let it melt. For a golden top, slide the skillet under the broiler for 60-90 seconds — watch closely. Season with salt, pepper, and red pepper flakes to taste. Garnish with fresh basil or parsley and serve immediately.