Begin by cleaning the mushrooms thoroughly. Place a large skillet on the stove over medium heat and add the mushrooms, garlic, and onion. Cover and simmer for 10-15 minutes until the mushrooms are tender.
Add marsala wine, vegetable stock, butter, sugar, pepper flakes, and Italian seasoning. Allow to boil for about 10 minutes or until the alcohol is cooked out. Tasting and smelling the sauce will help you determine when the alcohol has boiled out.
Combine corn starch and water in a separate container and mix until thoroughly combined. Add to the sauce pan and mix together. The sauce will thicken slightly.