Mix ground beef, breadcrumbs, and eggs together.
Add chopped onion and minced garlic.
Season with salt and pepper.
Mix everything until well combined.
Roll mixture into golf-ball sized meatballs.
You should get about 20-24 meatballs.
In a bowl, combine mushroom soup and broth.
Add Worcestershire sauce and onion soup mix.
Whisk until smooth and well blended.
Add sliced mushrooms to the mixture.
Stir everything together thoroughly.
Place meatballs in your slow cooker.
Arrange them in a single layer.
Pour gravy mixture over the meatballs.
Make sure all meatballs are covered.
The gravy should coat everything nicely.
Cover the slow cooker with its lid.
Cook on low for 6-7 hours.
Or cook on high for 3-4 hours.
The meatballs should be fully cooked.
The gravy will be thick and bubbly.