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.