Place 1/2 cup diced mushrooms in a large mixing bowl.
Add the ground beef, ground turkey, 1/2 cup diced onion, 2 Tablespoons tomato paste, egg, ground oatmeal, salt, pepper and 1/4 cup beef broth.
Mix together well and form into 1 1/2 inch meatballs; set aside.
In another bowl combine flour, remaining beef broth, remaining 1/4 cup onion, tomato paste, and Worcestershire sauce.
Heat oil in skillet over medium-high heat and brown the outside of the meatballs.
Place meatballs in the slow cooker sprayed with nonstick cooking spray.
Place remaining sliced mushrooms over top.
Pour sauce over all.
Cook on low for 6 hours.
Serve over mashed cauliflower and sprinkle with fresh parsley, if desired.