To a large bowl add the ricotta, egg, spinach, mozzarella, parmesan, salt, pepper, garlic, nutmeg, and lemon juice.
Stir everything together very well until well combined.
Set this mixture aside.
Preheat your oven to 400 degrees Fahrenheit (375 degrees Fahrenheit convection).
On a large cutting board or work surface, slice the chicken breasts to butterfly them by placing your hand on top of the chicken breast and slice the breast horizontally, leaving the top and bottom parts connected.
Open up the chicken breasts and divide the ricotta filling between the 6 chicken breasts.
Press the filling into the chicken and fold the top of the chicken over the bottom with the filling in the middle, securing the sides together with toothpicks. Repeat with all 6 chicken breasts.
Pour one cup of the marinara sauce into the bottom of a 9-inch by 13-inch baking dish and spread it out.
Place the stuffed chicken breasts into the bottom of the pan, nestling them into the marinara sauce.
Pour the remaining cup of marinara sauce over the chicken in the pan, and top with the shredded cheeses.
Bake for about 25-30 minutes at 400 degrees Fahrenheit (or until the internal temperature of the chicken measures 74 degrees Celsius or 165 degrees Fahrenheit).
After baking, let the chicken rest for about 5 minutes before serving.
This recipe is great for meal prep. Prepare as directed and place in an airtight container in the fridge for up to 3 days. Reheat well before serving.