In a large bowl, combine the ground beef, breadcrumbs, finely chopped onion, egg, garlic powder, and a generous pinch of salt and pepper. Mix with your hands until just combined — avoid overworking.

Shape the mixture into 12–16 golf-ball-sized meatballs (about 1½ inches).

Arrange the meatballs in a single layer in the slow cooker. They can touch — that’s fine.

In a medium bowl, whisk together the cream of mushroom soup, beef broth, and Worcestershire sauce until smooth.

Pour the soup mixture evenly over the meatballs. Scatter the sliced mushrooms on top.

Cover and cook on low for 6–8 hours, or on high for 3–4 hours. Meatballs are done when they reach 160°F (71°C) internal temperature and the gravy is bubbly and slightly thickened.

If the gravy is thin at the end, stir in a cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp cold water) and cook on high for 10–15 minutes to thicken.

Serve hot over mashed potatoes, rice, or noodles. Spoon plenty of gravy over each plate.