Boil a pot of water and cook 16 oz pasta according to the instructions. When done, drain the liquid and set the pasta aside.
Preheat the oven to 375F.
While the pasta is cooking, preheat a large skillet over medium/high heat and add 1 tbsp olive oil. When the oil is hot, add diced chicken, season it with salt and pepper, and sear until cooked through. When done, transfer the meat to a plate and set aside.
Lower the heat to medium/low. In the same skillet, melt 2 tbsp butter and add 2 tbsp all-purpose flour. Cook for a couple of minutes, then add 2 cups milk. Stir well and let it cook for about 10 minutes or until the mixture starts to simmer and get thicker.
Then add 28 oz crushed tomatoes, ½ tsp salt, ½ tsp dried oregano, ½ tsp dried thyme, and ¼ tsp red pepper flakes.
Cook for another 10 minutes, then add cooked pasta and mix everything until well combined. Then add precooked chicken with juices.
Fold in spinach and let it cook just for a couple of minutes or until the spinach is wilted.
Transfer everything to a 13×8 inch baking dish and top with 1 cup shredded mozzarella cheese.
Bake at 375F for about 10-15 minutes or until the the cheese is melted and starts to bubble and get light golden color.