In a mixing bowl, combine the cream of mushroom soup, beef broth, and onion soup mix. Stir until well blended.

Place the frozen meatballs in the bottom of the slow cooker. Add the sliced mushrooms and chopped onions on top of the meatballs.

Pour the prepared sauce over the meatballs and vegetables, ensuring everything is well coated.

Cover and cook on low for 6 hours or high for 3 hours, until the meatballs are heated through and the flavors meld.

In the last 30 minutes of cooking, add the frozen peas to the slow cooker and stir gently.

Once cooked, serve the Salisbury steak meatballs over mashed potatoes or rice, garnished with fresh parsley if desired.