01 -
In a large bowl, thoroughly mix ground beef, beaten egg, breadcrumbs, beefy onion soup mix, milk, and Worcestershire sauce using clean hands until evenly combined.
02 -
Divide mixture into 8 equal portions and shape each into a patty.
03 -
Place flour on a small plate and dredge both sides of each patty, tapping off excess flour.
04 -
Heat vegetable oil in a large skillet over medium-high heat. Sear patties in batches until golden brown on both sides, avoiding full cooking.
05 -
Spray a 6-quart oval slow cooker with cooking spray. Layer 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 until fully blended.
07 -
Pour gravy mixture over patties in the slow cooker. Cook on low for 4 to 5 hours or on high for 3 to 4 hours until internal temperature reaches at least 165°F. Longer cooking yields more tender patties.
08 -
Transfer patties to a plate. Bring the gravy in the slow cooker or a skillet to a simmer. Whisk together water and cornstarch in a small bowl to form a slurry. Gradually whisk slurry into the simmering gravy until thickened, about 1 to 2 minutes.
09 -
Return patties to the slow cooker if desired and pour thickened gravy over them. Keep warm until serving. Garnish with fresh chopped parsley if preferred.