Combine 1/4 cup of the cheddar cheese, the heavy cream, mozzarella cheese, cream cheese and chicken stock in a saucepan over medium-low heat. Heat, stirring frequently until the sauce is smooth and thick. Reduce the heat to low.
Heat the butter over medium heat in a large oven-proof skillet. Once the butter is melted and the pan is hot, add the broccoli, onion and garlic. Cook for 2 to 3 minutes. Add the spinach, sun-dried tomatoes, salt and pepper to the pan and cook until the spinach is wilted.
Add the vegetable mixture in with the cheese sauce and stir to combine.
Make slits in the sides of the chicken breasts to create a pocket to stuff the cheese and vegetable mixture into. Stuff the chicken breasts and arrange them in the skillet that you just cooked the vegetables in.
Pour the remaining cheese and vegetable mixture over top of the chicken breasts and then sprinkle them with the remaining cheddar cheese. Bake for 20 minutes.
Increase the temperature to broil and cook for an addition 5 minutes or until the chicken is golden brown and the cheese on top is crispy.