Prep your slow cooker: Start by placing the sliced mushrooms and onions at the bottom of your slow cooker. They’ll create a flavorful base for the patties and gravy.

Make the patties: In a medium bowl, mix ground beef, egg yolk, minced onion, panko bread crumbs, milk, garlic, salt, and pepper. Shape into 6 equal-sized patties.

Sear the patties: Heat a large skillet over medium-high heat and sear each patty for about 3 minutes per side. This step locks in flavor and keeps the patties intact during slow cooking.

Load the slow cooker: Layer the browned patties on top of the mushrooms and onions.

Prepare the gravy: In a bowl, whisk together the beef broth, brown gravy mix, ketchup, Dijon mustard, and parsley. Pour this mixture over the patties.

Cook: Cover and cook on low for 5 hours.

Thicken the gravy: Once cooked, carefully remove the patties. Turn the slow cooker to high. Mix the cornstarch with cold water, then stir it into the gravy. Let it cook a few minutes until thickened.

Finish and serve: Return the patties to the slow cooker, ensuring they’re well-coated in the gravy. Serve hot over mashed potatoes or rice.