Heat the oven to 400°F / 200°C (180°C fan).
Spread the marinara in the dish. Add the chicken in a single layer. Season with the Italian seasoning, garlic powder, plus a pinch of salt, and pepper.
24 ounces marinara sauce, 4 boneless skinless chicken breasts, 1 teaspoon Italian seasoning, ½ teaspoon garlic powder, salt, pepper
Squeeze the spinach dry in a towel or a fine-mesh sieve until most of the water is squeezed out. Mix with the ricotta, egg, and a pinch of salt.
10 ounces frozen spinach, 1 cup ricotta, 1 large egg, salt
Spread over each breast. Top with the mozzarella.
1 cup shredded mozzarella
Bake uncovered 30-35 min, until the chicken reaches 165°F / 74°C.
Rest 5 min. Spoon the sauce over each piece.