Prep your ingredients: Slice the sausage, chop the onions and garlic, and measure out your pasta and liquids.
Brown the sausage: Heat a large, deep skillet or Dutch oven over medium-high heat. Add a splash of oil and brown the sausage until golden.
Add aromatics: Toss in the chopped onions and garlic. Cook until softened and fragrant.
Pasta time: Add the pasta, 3 cups of chicken broth, the can of diced tomatoes, and 1 tbsp of tomato paste. Stir well.
Simmer: Bring to a boil, then reduce heat to low. Let it simmer uncovered, stirring occasionally, until the pasta is al dente and most of the liquid has been absorbed (about 12-15 minutes).
Cheese time: Stir in the mozzarella and Parmesan until melted and evenly distributed.
Final touches: Taste and adjust seasoning with salt and pepper. If using spinach, stir it in now and let it wilt for a minute. Serve warm and enjoy!