Stir breadcrumbs and milk in a bowl to make a panade; let stand 5 minutes.
Mix ground beef with the panade, minced onion, garlic powder, salt, pepper, and 1 1/2 tbsp Worcestershire until just combined.
Form the mixture into six oval patties, about 3/4 inch thick.
Heat oil in a large skillet over medium-high. Brown patties 1–2 minutes per side; do not cook through.
Spread sliced onion and mushrooms in the slow cooker.
Whisk broth, ketchup, tomato paste, Dijon, and 1 tbsp Worcestershire; pour over the onions and mushrooms.
Nestle the browned patties into the gravy. Cover and cook on LOW for 4 hours.
Stir cornstarch with water to make a slurry. Stir it into the cooker, cover, and cook until thickened, about 10–15 minutes.
Taste and adjust salt and pepper. Garnish with parsley and serve with mashed potatoes or buttered noodles.
Notes
For a richer gravy, add a splash of cream during the final thickening step. Prefer turkey? Swap in ground turkey and use chicken broth. Leftovers keep well in the fridge up to 3 days; reheat gently so the gravy stays silky.This recipe is an original creation inspired by classic Easy Slow Cooker Salisbury Steak Recipe flavors. All ingredient ratios and instructions are independently developed.