In a mixing bowl, combine the ground beef, breadcrumbs, chopped onion, minced garlic, egg, Worcestershire sauce, salt, and black pepper. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter. Set aside.
In a large skillet over medium heat, brown the meatballs on all sides until they form a nice golden crust, about 5-7 minutes. Transfer the meatballs to the slow cooker.
In the same skillet, add the sliced mushrooms and sauté for about 3-4 minutes until they begin to soften.
Add the beef broth and onion soup mix to the skillet. Stir to combine and bring to a simmer for 2 minutes.
Carefully pour the mushroom and broth mixture over the meatballs in the slow cooker.
Cover and cook on low for 6 hours or until meatballs are cooked through and tender.
If you prefer a thicker gravy, mix the cornstarch with water in a bowl until smooth. Stir into the crockpot during the last 30 minutes of cooking.
Serve the meatballs and gravy over mashed potatoes, rice, or noodles.