Sausage, Mozzarella, and Spinach with Shells

My mom and I whipped up this comfort food the other night, which was perfect on a cold Michigan evening. I found this recipe in Everyday Food, by Martha Stewart, but we changed it up a bit. Her recipe called for broccoli rabe, but we substituted spinach. Make this when you want to curl up on the couch, watch a movie, and have a filling pasta dish!

2 T extra virgin olive oil

1 large onion, chopped

3 sprigs thyme (I used dry)

salt and pepper

2 garlic cloves, chopped

3 links spicy Italian sausage(use turkey sausage for a healthier dish), casing removed

1 T all purpose flour (I used white whole wheat flour)

1 can whole peeled tomatoes (28 oz)

8 oz. whole wheat medium shells

1 lb. baby spinach

6 oz. fresh mozzarella, cut into 1/2 inch pieces

1/4 cup grated parmesan

1. Preheat oven to 400. In a large skillet, heat oil over medium high. Add onion and thyme; season with salt and pepper. Cook, stirring occasionally, until onion is golden brown, 15 minutes.
2. Add garlic and sausage. Cook, breaking meat up with a wooden spoon, until browned, about 5 minutes. Add flour and cook, stirring occasionally, 30 seconds. Add tomatoes, breaking them up with spoon. Cook sauce until slightly thickened, 5 to 8 minutes. Season with salt and pepper. ( If using fresh thyme, discard now)
3. Meanwhile, in a large pot of boiling water, cook pasta 4 minutes less than package instructions. Add spinach to pot and cook 15 seconds. Drain pasta and spinach, and return to pot. Stir in sausage mixture. Transfer to a 3 quart baking dish or divide among four 16 oz. gratin dishes. Top with mozzarella and parmesan. Bake until cheese has melted and liquid is bubbling, about 15 minutes.