Sauté Onion and Garlic: Heat 1 tablespoon of olive oil in a skillet over medium heat. Toss in the diced onion and sauté for 3-4 minutes, or until tender. Then, add the garlic and cook for an additional 30 seconds, just until it becomes fragrant.

Add Spinach: Stir in the fresh spinach and cook until wilted, about 1-2 minutes. Take the mixture off the heat and allow it to rest for a few minutes to cool down slightly.

Combine Cheeses and Spinach: In a bowl, mix the softened Boursin cheese, shredded mozzarella, Italian seasoning, and red pepper flakes (if using) with the sautéed spinach, onion, and garlic. Stir until well combined.

Preheat Oven: Preheat your oven to 375°F (190°C).

Slice Pockets in Chicken Breasts: Cut a horizontal pocket in each chicken breast, being careful not to cut all the way through.

Stuff the Chicken: Season each chicken breast with salt and pepper. Fill each pocket with an even amount of the cheese and spinach mixture, pressing gently to seal.

Sear the Chicken: In an oven-safe skillet, heat the remaining 2 tablespoons of olive oil over medium-high heat. Brown the stuffed chicken breasts in a hot pan for 2-3 minutes on each side, until they achieve a rich golden color.

Bake: Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).

Serve Hot: Let the chicken rest for a few minutes before slicing. Serve with a side of vegetables, mashed potatoes, or a fresh salad.