Preheat your oven to 375°F (190°C). Butterfly the medium chicken breasts by slicing them horizontally, creating a pocket for stuffing. Season generously with salt and pepper, then lightly dredge each breast in all-purpose flour, shaking off any excess.

In a large skillet over medium heat, add 2 tablespoons of olive oil. Once the oil shimmers, add minced garlic and sauté until fragrant, about 1 minute. Gradually add fresh baby spinach, tossing until wilted, about 2-3 minutes. Mix in the sliced roasted red peppers, remove from heat, and set aside to cool slightly.

Place a slice of fresh mozzarella inside each seasoned chicken breast pocket, followed by a generous spoonful of the spinach and red pepper mixture. Fold the chicken over to enclose the filling completely. Secure the edges with toothpicks if needed.

In the same skillet, add another tablespoon of olive oil and 1 tablespoon of butter, heating over medium-high heat. Gently place the stuffed chicken breasts in the skillet, searing for 5-6 minutes on each side until golden brown and the internal temperature reaches 165°F (74°C).

Transfer the stuffed chicken breasts to a plate and let them rest for about 5 minutes. Slice the chicken to reveal the vibrant filling and serve immediately, perhaps garnished with fresh herbs.