Step 1: Layer the Aromatics and Chicken
In your slow cooker, combine the diced onion, minced garlic, diced tomatoes (with their juices), chicken broth, tomato paste, dried Italian seasoning, basil, oregano, red pepper flakes (if using), salt, and black pepper. Whisk these base ingredients together directly in the crock until well combined. This creates the flavorful broth in which everything will cook. Place the raw chicken thighs (or breasts) evenly on top of this saucy mixture, submerging them slightly. Pour the entire jar of marinara sauce over the chicken, spreading it to cover.

Step 2: The Long, Slow Cook
Cover the slow cooker with its lid. 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 reaches an internal temperature of 165°F (74°C). The low and slow method on the LOW setting is highly recommended for the most tender, shreddable chicken.

Step 3: Shred the Chicken
Using two forks, carefully shred the chicken directly in the slow cooker. It should pull apart with minimal effort. Stir the shredded chicken back into the sauce until evenly distributed.

Step 4: Cook the Pasta and Create Creaminess
Add the uncooked, dry pasta to the slow cooker, stirring to ensure all pieces are submerged in the liquid. If the mixture seems too thick, you can add up to ½ cup more chicken broth or water. 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. It will continue to cook in the residual heat.

Step 5: The Creamy, Cheesy Enrichment
Once the pasta is cooked, turn the slow cooker to the WARM setting. Stir in the heavy cream (or half-and-half) and the ½ cup of grated Parmesan cheese until fully incorporated. The sauce will transform into a lovely, creamy pink-orange hue. Taste and adjust seasoning with more salt, pepper, or red pepper flakes as needed.

Step 6: The Melty, Golden-Brown Top
Preheat your oven’s broiler to high. Carefully transfer the entire contents of the slow cooker to a large, oven-safe baking dish (or if your slow cooker crock is oven-safe, you can use it directly). Evenly sprinkle the top with the remaining 1 cup of shredded mozzarella and ¼ cup of Parmesan cheese. 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 7: Rest, Garnish, and Serve
Remove the dish from the oven and let it rest for 5 minutes. This allows the sauce to thicken slightly and makes serving easier. Garnish generously with freshly chopped basil or parsley. Serve immediately in deep bowls, with extra grated Parmesan on the side and plenty of garlic bread for dipping into the luxurious sauce.