To make the sauce: Coat a large nonstick pan with cooking spray. Heat olive oil in pan over medium-high heat. Add onions, yellow peppers and mushrooms and cook about 4-5 minutes, until soft. Stir often. Add garlic, tomato paste, red pepper flakes and cook for 1 minute. Stir in tomatoes, vinegar, Italian seasonings and sugar. Cook until slightly thickened, about 3-5 minutes. Stir often. Reduce to low heat to keep warm.