. Preheat the oven to 350°F (180°C). Lightly oil a baking sheet or use an oven-safe skillet.
Whisk the Dijon mustard, vinegar, sugar, olive oil, Italian seasoning, red pepper flakes, salt, and black pepper in a bowl.
Cut a deep horizontal pocket into each chicken breast, stopping before cutting all the way through.
Coat the chicken inside and out with the Italian dressing.
Fill each chicken breast with sun-dried tomatoes, mozzarella, and spinach. Fold closed and secure with toothpicks if needed.
Heat olive oil in a skillet over medium-high heat. Sear the chicken for 1 1/2 minutes per side until lightly golden.
Transfer to the oven and bake for about 15 minutes, until the chicken reaches 165°F (74°C) in the thickest part.
Rest for 3 minutes, remove toothpicks, then spoon the pan juices over the chicken before serving.