Make the Patties:
In a large bowl, combine ground beef, breadcrumbs, egg, ketchup, Worcestershire sauce, garlic powder, black pepper, and half of the onion soup mix. Mix until combined, then shape into 6–8 oval patties.

Brown the Patties (Optional):
For extra flavor, sear patties in a skillet over medium-high heat for 2–3 minutes per side until lightly browned. (Skip if in a rush.)

Layer in the Slow Cooker:
Add sliced onions to the bottom of the slow cooker. Layer the beef patties over the onions.

Make the Gravy:
In a bowl, whisk the beef broth with the remaining onion soup mix. Pour the mixture over the patties.

Cook:
Cover and cook on low for 5–6 hours or high for 3–4 hours, until patties are cooked through and tender.

Thicken Gravy (Optional):
For a thicker gravy, mix 2 tablespoons cornstarch with 2 tablespoons cold water. Stir into the slow cooker and cook on high for an additional 15–20 minutes.

Serve:
Serve patties warm with gravy over mashed potatoes, egg noodles, or rice. Garnish with parsley if desired.