Prepare the spinach filling: Add the olive oil, chopped onion, and a pinch of salt to a skillet over medium heat. Cook until translucent and lightly browned.
Add the fresh spinach and sauté for a few seconds until slightly wilted.
Stir in the minced garlic and cook briefly until fragrant.
Transfer the mixture to a bowl. Add the ricotta, nutmeg, cheeses, salt, and pepper. Mix well until everything is combined. Set aside.
Prepare the chicken: Pat the chicken breasts dry with paper towels. Using a knife, make several slits in the top of each breast, being careful not to cut all the way through (to create small pockets).
Stuff the chicken: Using a spoon, place a small amount of filling into each slit in the chicken breast.
Bake: Place the stuffed chicken breasts in a baking dish. Season with salt and pepper, and drizzle lightly with olive oil. 8. Bake at 200°C (400°F) for 25-30 minutes, or until the chicken is cooked through and golden brown.