To a large dutch oven on medium heat, add the olive oil and heat it until it shimmers.
Add the hot Italian sausage, cook until the the first side browns deeply, flip and cook on the other side until the sausage is fully cooked, 15 minutes. Remove the sausages from the pan and slice when they cool down.
To the leftover pan drippings, add the chopped yellow onion, minced garlic, salt, pepper, dried basil, dried oregano, and dried parsley. Cook until the onions are caramelized and soft. About 10 minutes.
Add the tomato paste and cook until the tomato paste darkens slightly.
Add in the diced tomatoes and the sausage slices into the pan, stir, and let the sauce come to a simmer.
Add in half the grated parmesan, spinach, the cooked pasta, and stir to combine.
Spoon in fresh dollaps of the ricotta cheese, garnish with fresh basil, more grated parmesan, and serve!