In a large mixing bowl, combine ground beef, panko breadcrumbs, beaten egg, Worcestershire sauce, Dijon mustard, tomato paste, onion powder, garlic powder, salt, and black pepper. Mix gently with your hands just until evenly combined—do not over-compact.

Divide beef mixture into 6 equal portions and shape each into an oval patty about 3/4-inch thick. Heat olive oil in a large cast-iron skillet over medium-high heat. Sear the patties for 2 to 3 minutes per side until a deep golden-brown crust forms (they do not need to be cooked through).

Scatter half of the sliced onions and sliced mushrooms across the bottom of a 6-quart slow cooker. Transfer the seared beef patties into the slow cooker in an even layer. Top with the remaining sliced onions and mushrooms.

In a medium measuring cup, whisk together beef broth, Worcestershire sauce, soy sauce, and dried thyme. Pour the broth mixture evenly over the patties and mushrooms in the crockpot.

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.

Carefully transfer the cooked patties to a warm plate and cover loosely with foil. Turn slow cooker to HIGH. In a small bowl, whisk cornstarch and cold water until smooth. Pour the slurry into the bubbling broth and stir gently for 3 to 5 minutes until thickened into a glossy, velvety brown gravy.

Return beef patties to the slow cooker and spoon the rich mushroom gravy all over them. Garnish with chopped fresh parsley and serve hot over creamy garlic mashed potatoes or buttered egg noodles!