Stuffed Chicken Breast with Mozzarella and Spinach

boneless skinless chicken breasts, butterflied

 tablespoons olive oil, divided

slices of fresh mozzarella cheese

cups fresh baby spinach

 cloves garlic, minced

small onion, minced

low-sodium crushed tomatoes

sugar

balsamic vinegar

chicken stock

Crushed red pepper flakes, to taste (optional)

Salt and fresh cracked pepper