Ingredients

• Italian sausage - about 1.25 lb, sliced
• Olive oil - 2 tbsp, divided
• Yellow onion (sliced) - 1 large
• Yellow onion (diced) - 1 small
• Garlic - 8 cloves, minced or pureed
• Tomato sauce - 1 (28 oz) can
• Salt - to taste
• Black pepper - to taste
• Red pepper flakes - 1/2 tsp, or to taste
• Italian seasoning - 1 tsp
• Spaghetti - 12 oz dry
• Parmesan cheese - for serving
• Garlic bread - for serving (optional)

Steps

1. Heat {{Olive oil:1 tbsp}} in a large skillet or griddle over medium-high heat. Add {{Italian sausage:1.25 lb, sliced}} and {{Yellow onion (sliced):1 large}}; cook, stirring occasionally, until the sausage is browned and cooked through and the onions are caramelized, 8–10 minutes. Set aside.
2. In a saucepan, heat {{Olive oil:1 tbsp}} over medium heat. Add {{Yellow onion (diced):1 small}}, {{Salt:pinch}}, and {{Black pepper:pinch}}; cook until softened, 3–5 minutes.
3. Stir in {{Garlic:8 cloves}} and cook until fragrant, 30–60 seconds.
4. Pour in {{Tomato sauce:28 oz}}. Season with {{Salt:to taste}}, {{Red pepper flakes:1/2 tsp}}, and {{Italian seasoning:1 tsp}}. Stir and simmer gently for 20 minutes.
5. Blend the sauce with a stick blender until smooth, then simmer 10 minutes more. Adjust {{Salt:to taste}} and {{Black pepper:to taste}} if needed.
6. Bring a large pot of water to a boil and season with {{Salt:1 tbsp}}. Cook {{Spaghetti:12 oz}} until al dente, then drain.
7. To serve, divide the cooked {{Spaghetti:as desired}} among plates. Spoon over some {{Italian sausage:as desired}} along with the cooked {{Yellow onion (sliced):as desired}}, then ladle on the marinara.
8. Finish with {{Parmesan cheese:as desired}} and serve with {{Garlic bread:as desired}}.