Step 1: Build the Flavor Foundation
In your slow cooker, combine the diced onion, minced garlic, diced tomatoes (with their juices), tomato paste, Italian seasoning, basil, oregano, red pepper flakes (if using), salt, and black pepper. Stir these base ingredients together. Place the raw chicken on top of this mixture. Pour the entire jar of marinara sauce over the chicken, spreading it to cover.

Step 2: The Long, Slow Simmer
Cover and cook on LOW for 6-7 hours or on HIGH for 3-4 hours. The chicken is done when it is fall-apart tender and shreds easily with a fork. If using chicken breasts, check 1 hour earlier as they cook faster and can dry out.

Step 3: Shred the Chicken
Using two forks, carefully shred the chicken directly in the slow cooker into bite-sized pieces. Stir the shredded chicken back into the sauce until evenly distributed.

Step 4: Cook the Pasta
Add the uncooked, dry pasta to the slow cooker, stirring to ensure all pieces are submerged in the sauce. If the mixture seems too thick, add up to ½ cup of water or chicken broth. Cover and cook on HIGH for an additional 25-35 minutes. Start checking at 25 minutes—the pasta should be al dente, tender but with a slight bite.

Step 5: Enrich and Prepare for Topping
Once the pasta is cooked, stir in the heavy cream (if using) and ½ cup of the grated Parmesan cheese. This adds a lovely richness and helps the sauce cling to the pasta. Preheat your oven’s broiler to high.

Step 6: Add Cheese and Broil to Perfection
Carefully transfer the entire contents of the slow cooker to a large, oven-safe baking dish (like a 9×13 casserole). Evenly sprinkle the top with the shredded mozzarella and the remaining ½ cup of Parmesan cheese.

Step 7: The Golden-Brown Finish
Place the dish under the broiler, watching closely, for 2-4 minutes, or until the cheese is completely melted, bubbly, and spotted with golden brown. CAUTION: Broilers work very fast. Do not walk away!

Step 8: Rest, Garnish, and Serve
Remove the dish from the oven and let it rest for 5-10 minutes. This allows the sauce to thicken slightly and makes serving easier. Garnish generously with fresh chopped basil or parsley. Serve immediately in bowls, with extra grated Parmesan on the side and plenty of garlic bread for dipping