Pat chicken breasts dry and cut a horizontal slit in each about ⅔ through. Season with salt and pepper and dredge each breast in flour, shaking off excess.

In a large oven-safe skillet, heat 1 tablespoon oil and butter over medium-high heat until shimmery. Sear chicken on both sides 2-3 minutes per side until golden brown. Transfer to plate and set aside.

In the same skillet over medium-high heat, sauté roasted red peppers and spinach in remaining 1 tablespoon olive oil until spinach is wilted, about 3-5 minutes. Add garlic and sauté for 1 additional minute until fragrant. Season with salt and pepper to taste. Transfer filling to a small bowl.

Return chicken to skillet off-heat. Fill each chicken breast with a slice of mozzarella cheese and roasted red pepper mixture using toothpicks to secure. Bake at 375F for 20-25 minutes or until chicken registers 165F. Remove toothpicks before serving.