In a 3 1/2- to 4-quart slow cooker combine mushrooms, sweet pepper, onion, Italian seasoning, salt, and black pepper. Pour marinara sauce over all. Using a wooden spoon break up ground turkey into bite-sized pieces. Add to cooker, stirring to combine.
Cover and cook on low-heat setting for 3 hours.
In a small bowl combine baking mix, Parmesan cheese, and oregano. Add milk; stir with a fork until combined.
Turn cooker to high-heat setting. Drop dumpling mixture by tablespoons into five mounds onto mixture in cooker, spacing mounds evenly.
Cover and cook for 45 to 60 minutes more or until a toothpick inserted into the centers of dumplings comes out clean.
To serve, if desired, sprinkle each serving with mozzarella cheese.