Spray or grease the inside of your slow cooker or crock pot. Season each chicken breast with salt and pepper on each side and place into the slow cooker in one layer.
Using a large mixing bowl, pour in the marinara sauce, and add the minced garlic and dried oregano and crushed red pepper flakes if using. Mix together, taste and add salt and pepper if desired. Pour the sauce over the chicken breasts into the slow cooker. Make sure all of the chicken is covered with the sauce. Cover and cook on LOW for about 4 hours.
After the chicken is cooked, sprinkle on the panko over each chicken breast. Alternatively, you can brown the panko or bread crumbs in a pan before sprinkling over the chicken. Add the shredded mozzarella cheese and parmesan over each chicken breast and cover the slow cooker and let cook until the cheese has melted. Garnish with the fresh parsley and parmesan cheese and serve over cooked spaghetti.