Cook noodles until al dente and then rinse and set aside. Sauté chicken pieces and onions in the butter in a large skillet until golden brown and mostly cooked through--about 5-7 minutes. Season chicken with salt and pepper. Add the balsamic vinegar, sugar and Italian seasoning to chicken and onions and simmer until sauce begins to thicken. Add the mozzarella cheese and fresh tomatoes to sauce and chicken. Continue to simmer for another 2-3 minutes. Pour cooked and drained pasta into the sauce and serve with extra cheese on top. Garnish with fresh basil (optional).