Salt a pot of water and bring to a boil. Add the pasta and cook until al dente, about 6-8 minutes.
Drain pasta and set aside.
Remove sausage from casings and heat in a skillet over medium heat.
Break up the sausage into crumbled pieces and cook until browned, about 10 minutes. Remove from skillet.
Add onion and garlic to the skillet and cook just until softened and fragrant, about 2-3 minutes.
Add tomatoes to the skillet and cook until tomatoes break down. Add herbs, salt and pepper, and red pepper flakes.
Place spinach in the pan and cook just until wilted.
Then, add the sausage and pasta into the skillet and stir to combine.
Spoon the pasta mixture into a casserole dish.
Top with mozzarella and cook in a 350 degree oven for 20 minutes, or until cheese is melted.
Serve immediately.