Add the diced onion, minced garlic, and diced bell pepper to the slow cooker. Stir to combine.

Add the chicken pieces to the slow cooker. Stir to coat with the vegetables.

Pour the marinara sauce, chicken broth, and tomato paste over the chicken and vegetables. Stir to combine.

Sprinkle the Italian seasoning, basil, oregano, and red pepper flakes over the mixture. Stir to combine.

Cover the slow cooker and set it to low. Cook for 6 to 7 hours, or until the chicken is tender and easily shreds with a fork.

Remove the chicken from the slow cooker and shred it using two forks. Return the shredded chicken to the slow cooker.

Add the sliced mushrooms to the slow cooker. Stir to combine.

Cover the slow cooker and cook for an additional 30 minutes, or until the mushrooms are tender.

Cook the pasta according to package instructions until al dente. Drain and set aside.

Stir the heavy cream, ricotta cheese, and parmesan cheese into the soup. Stir until the cheese is melted and the soup is smooth.

Add the cooked pasta to the soup. Stir to combine.

Sprinkle the shredded mozzarella cheese over the top of the soup.

Cover the slow cooker and cook for an additional 10 minutes, or until the mozzarella cheese is melted and the soup is heated through.