1. 1. Prepare the Chicken and Aromatics: Place the boneless, skinless chicken breasts at the bottom of your slow cooker. Scatter the chopped onion and minced garlic evenly over the chicken.

2. 2. Add the Sauce and Seasonings: Pour the entire jar of marinara sauce and the can of undrained diced tomatoes over the chicken and aromatics. Sprinkle the dried oregano, dried basil, and red pepper flakes (if using) over the sauce. Pour in the chicken broth.

3. 3. Slow Cook to Perfection: Cover your slow cooker with the lid and cook on high for 3-4 hours or on low for 6-8 hours. The chicken should be very tender and easily shredded with a fork when it’s done.

4. 4. Shred the Chicken: Once the chicken is cooked through, carefully remove it from the slow cooker and place it on a cutting board. Using two forks, shred the chicken into bite-sized pieces. Return the shredded chicken to the slow cooker, stirring it into the sauce.

5. 5. Stir in the Pasta: Add the uncooked penne pasta directly into the slow cooker with the shredded chicken and sauce. Stir well to ensure the pasta is fully submerged in the liquid.

6. 6. Finish Cooking the Pasta: Cover the slow cooker again and continue to cook on high for an additional 20-30 minutes, or until the pasta is al dente (tender but still firm to the bite). Keep an eye on it to prevent overcooking.

7. 7. Add the Cheese: Once the pasta is cooked, sprinkle the shredded mozzarella cheese and grated Parmesan cheese over the top of the pasta mixture. Replace the lid for about 10-15 minutes, or until the cheese is melted and bubbly. You’ll see a beautiful, gooey layer of cheese forming.

8. 8. Serve and Garnish: Ladle generous portions into bowls. If desired, garnish with fresh chopped parsley for a touch of color and freshness. Serve hot and enjoy!