Bring a large pot of salted water to a rolling boil. Add the spaghetti and cook according to the lower end of the package time for al dente, about 8–10 minutes. Reserve 1 cup of the pasta cooking water before draining.

Drain the pasta and set aside while you finish the sauce.

Heat a large skillet over medium heat and add 1 to 2 tablespoons of olive oil.

Add the chopped onion and sauté until translucent, about 4–6 minutes. Add minced garlic and cook for 30–45 seconds until fragrant.

Add the Italian sausage and brown it for about 6–8 minutes, breaking it into small pieces as it cooks.

If there’s excess fat, spoon out most of it. Pour in the crushed tomatoes and sprinkle in Italian seasoning. Season with salt and pepper. Let simmer for 8–12 minutes.

Adjust the seasoning as needed. If too acidic, stir in a pinch of sugar or butter.

Add the drained spaghetti to the skillet with sauce. Toss to coat and loosen with reserved pasta water if necessary. Cook for an additional 1–2 minutes.

Divide among plates and top with Parmesan cheese, olive oil, red pepper flakes, or fresh herbs if desired.