In a medium saucepan over medium heat, add oil and when hot, add onions and celery; sauté for 3 minutes. Add red peppers and sauté for 2 minutes. Stir in mushrooms and cook for 4 minutes. Add garlic and quickly sauté for 1 minute. Add cooked turkey, Marinara sauce, Italian seasoning and sharp Cheddar cheese; stir until the ingredients are well blended.