Season the chicken with the salt and black pepper and brown it in the olive oil for 3 minutes per side.
Stir the crushed tomatoes, garlic, basil, oregano, crushed red pepper and Parmesan in a 6-quart slow cooker.
Nestle the chicken in the sauce, cover and cook on low until the breasts reach 165°F, 3 to 3 1/2 hours.
Turn off the cooker, sprinkle the mozzarella evenly over the chicken and replace the lid.
Let the cheese melt in the residual heat for 10 minutes.
Slice the chicken and serve it with the tomato sauce over freshly cooked spaghetti.