In a large skillet over medium heat, cook the onion in a splash of oil for 5-6 minutes, until softened and beginning to brown.
Add the sausage to a skillet. I like to push it out of the casing in small portions, like mini meatballs. Once the meatballs are browned, after about 4-5 minutes, add the basil, oregano, red pepper flakes, and garlic and season with salt and pepper to taste. Remove from heat.
In the casserole dish, mix the crushed tomato, chicken stock, and milk. Season with salt and pepper.
Add the uncooked pasta. Make sure the pasta is submerged.
Add the browned sausage and onions.
Cover tightly with foil. Bake for 40 minutes.
Remove the foil carefully and add the spinach, stirring the dish well. Add the shredded cheese.
Broil for 2-3 minutes until the cheese is browned and bubbling or bake for 5 minutes until the cheese is melted.
Video
Notes
The pasta needs enough sauce and liquid to cook properly. Stick to the ratios for best results.Cover tightly with foil: Sealing the dish helps trap the steam and cook the pasta evenly.Stir when adding the spinach and cheese. This helps the pasta at the top of the baking dish soften a bit more.Cool the pasta bake completely and store it in an airtight container in the refrigerator for up to 4 days.