Mix Patties: In a large bowl, combine ground beef, breadcrumbs, eggs, onion, garlic, Worcestershire sauce, Dijon mustard, salt, and pepper. Mix gently until just combined.
Form Patties: Shape mixture into 6 oval patties.
Optional Browning: Heat a skillet with a little oil and sear patties on both sides until browned (about 2 minutes per side). This step adds flavor and helps patties hold together but is optional.
Layer Ingredients in Crockpot: Place sliced onions and mushrooms at the bottom. Lay patties on top.
Make Gravy Base: In a bowl, whisk together beef broth, ketchup, Worcestershire sauce, and soy sauce. Pour over patties.
Slow Cook: On Low: 4–6 hours,On High: 2–3 hours
Thicken Gravy: About 20 minutes before serving, stir in a cornstarch slurry. Let it cook until thickened.
Serve Hot: Spoon gravy over patties and pair with your favorite sides.