Lightly coat the inside of a 6-quart slow cooker with cooking spray.
In a shallow dish, whisk the egg.
In a second shallow dish, combine the breadcrumbs, Parmesan cheese, salt, and pepper. Mix well.
Dip each chicken breast into the whisked egg, letting any excess drip off. Then, press the chicken into the breadcrumb mixture, coating both sides.
Arrange the coated chicken breasts in a single layer in the bottom of the prepared slow cooker. You may need to overlap them slightly.
Pour the marinara sauce evenly over the chicken. Sprinkle with 1 cup of the mozzarella cheese.
Cover and cook on LOW for 4 to 5 hours, or until the chicken is cooked through and tender. The internal temperature should reach 165°F (74°C) when checked with a meat thermometer.
Sprinkle the remaining 1 cup of mozzarella cheese over the top. Cover and cook for another 10 minutes, or until the cheese is melted and bubbly.
While the cheese melts, cook the spaghetti according to package directions. Serve the chicken parmigiana over the cooked pasta, garnished with fresh basil or parsley if desired.