01 -
Combine ground beef, egg, breadcrumbs, onion soup mix, milk, and Worcestershire sauce in a large bowl. Mix thoroughly using clean hands until evenly combined.
02 -
Divide the meat mixture into 8 equal portions and shape each into a patty.
03 -
Pour flour onto a shallow plate. Dredge both sides of each patty in flour, tapping off excess.
04 -
Heat vegetable oil in a skillet over medium-high heat. Sear patties in batches until golden brown on both sides, avoiding full cooking.
05 -
Spray a 6-quart or similar slow cooker with cooking spray. Layer the seared patties inside, stacking if necessary.
06 -
In a large bowl, whisk together beef broth, au jus gravy mix, minced garlic, Worcestershire sauce, and Dijon mustard.
07 -
Pour the prepared gravy mixture evenly over the patties in the slow cooker.
08 -
Cook on low for 4 to 5 hours or on high for 3 to 4 hours until patties reach an internal temperature of at least 165°F (74°C). Longer cooking time improves tenderness.
09 -
Remove patties and transfer gravy to a large skillet. Bring to a simmer. Whisk water and cornstarch together; gradually add this slurry into the simmering gravy while whisking. Continue simmering 1 to 2 minutes until thickened.
10 -
Return patties to the slow cooker and pour the thickened gravy over them. Keep warm up to 2 hours before serving. Garnish with fresh chopped parsley if desired.