In large nonstick skillet, brown sausage over medium-high heat. Stir in onion, spinach and mushrooms. Cook, stirring occasionally, 5 minutes or until sausage is done. Stir in sauce; heat through.
For each sandwich, split open each bread piece, and evenly spoon in sausage mixture. Sprinkle, if desired, with crushed red pepper flakes.
Find recipes with ingredients that you have on hand.