In a large bowl, combine ground beef, eggs, milk, breadcrumbs, minced onion, garlic, salt, pepper, and 1 tsp Worcestershire sauce. Mix gently with clean hands or a spoon until just combined — do not overmix.
Scoop about 1.5 tablespoons of meat mixture per meatball and roll between your palms into smooth balls; you should get approximately 30–36 meatballs. Set aside on a plate or baking sheet.
In a medium bowl, whisk together the cream of mushroom soup, dry onion soup mix, beef broth, 2 tbsp Worcestershire sauce, garlic powder, onion powder, and black pepper until smooth and pourable.
Spray the crockpot insert with cooking spray, then spread half the sliced mushrooms on the bottom. Arrange meatballs in as close to a single layer as possible, scatter remaining mushrooms over the top, and pour the gravy evenly over everything.
Cover and cook on HIGH for 4 hours or LOW for 6 hours, keeping the lid on throughout. In the last 30 minutes, gently stir to redistribute the gravy; meatballs are done when they reach an internal temperature of 160°F (71°C).
If you prefer thicker gravy, stir in a slurry of 2 tbsp cornstarch mixed with 2 tbsp cold water, then cook on HIGH for an additional 15–20 minutes, stirring occasionally, until thickened.
Taste the gravy and adjust seasoning as needed, then use a slotted spoon to serve meatballs over mashed potatoes, egg noodles, or rice, spooning plenty of mushroom gravy on top. Garnish with fresh parsley if desired.