Mix the Patty Base: In a large mixing bowl, gently combine ground beef, panko breadcrumbs, egg, Worcestershire sauce, Dijon mustard, ketchup, garlic powder, onion powder, kosher salt, and black pepper. Mix with your hands just until combined—do not overwork the meat to ensure tender patties.

Shape the Patties: Divide the mixture into 6 equal portions and shape each into an oval patty, approximately 1/2-inch to 3/4-inch thick. Press a slight dimple in the center of each patty to prevent puffing during cooking.

Sear for Crust: Heat olive oil in a large heavy skillet over medium-high heat. Sear the patties in batches for 2 to 3 minutes per side until a deep golden-brown caramelized crust develops. (Note: The patties do not need to cook through here; searing locks in moisture and deep savory flavor).

Layer in Slow Cooker: Scatter half of the sliced onions and sliced mushrooms across the bottom of a 6-quart slow cooker. Transfer the seared beef patties on top of the vegetables in an even layer. Scatter the remaining onions and mushrooms over the patties.

Whisk the Gravy: In a medium bowl or glass measuring cup, whisk together beef broth, brown gravy mix, tomato paste, Worcestershire sauce, and minced garlic until smooth. Pour the gravy mixture evenly over the patties and vegetables in the crockpot.

Slow Cook to Perfection: Cover with the lid and cook on LOW for 4 to 5 hours (or on HIGH for 2.5 to 3 hours) until the beef patties are melt-in-your-mouth tender and cooked through.

Thicken & Serve: If you prefer an extra thick, glossy gravy, remove patties to a warm plate, stir the cornstarch slurry into the slow cooker, and cook on HIGH uncovered for 10 to 15 minutes until velvety. Return patties, spoon rich mushroom gravy generously over each, garnish with chopped parsley, and serve hot over mashed potatoes!