Step 1: Butterfly each chicken breast: Lay flat, slice horizontally through the thickest part, stopping an inch from the end. Open like a book. Place plastic wrap over and pound to 1/2-inch thickness. Season both sides with salt and pepper.
Step 2: In a medium bowl, combine ricotta cheese, 1/2 cup Parmesan cheese, chopped spinach, minced garlic, a pinch of salt, pepper, and nutmeg. Mix well.
Step 3: Spoon a generous amount of the ricotta mixture onto one half of each pounded chicken breast, leaving a small border. Fold the other half over the stuffing, or roll it up, securing with toothpicks if needed, to enclose the filling.
Step 4: Set up three shallow dishes: one with flour, one with whisked eggs, and one with Panko breadcrumbs mixed with Italian seasoning, garlic powder, and onion powder.
Step 5: Dredge each stuffed chicken breast in flour, then dip in egg, and finally coat thoroughly in the seasoned Panko breadcrumbs, pressing gently to adhere.
Step 6: Heat olive oil in a large oven-safe skillet over medium-high heat. Sear the breaded chicken for 3-4 minutes per side until golden brown and crispy. Work in batches if necessary to avoid overcrowding.
Step 7: Preheat oven to 375°F (190°C). Pour marinara sauce over and around the chicken in the skillet. Top each breast with mozzarella slices and a sprinkle of extra Parmesan. Bake for 20-25 minutes, or until chicken is cooked through (165°F / 74°C internal temperature) and cheese is bubbly and golden. Remove toothpicks before serving.