In a large bowl, gently combine the ground beef, Panko breadcrumbs, egg, 2 tablespoons of the onion soup mix, Worcestershire sauce, garlic powder, salt, and pepper. Mix with your hands until just combined, but do not overmix.

Shape the mixture into 4-6 oval patties, about 3/4-inch thick.

Heat olive oil in a large skillet over medium-high heat. Sear the patties for 2-3 minutes per side until a brown crust forms. They will not be cooked through. Transfer the seared patties to the bottom of a 6-quart slow cooker.

To the same skillet, add the sliced onions and mushrooms. Sauté for 5-7 minutes until softened and browned. Add the minced garlic and cook for 1 minute more.

Sprinkle the flour over the vegetables and stir continuously for 1 minute.

Slowly whisk in the beef broth, scraping the bottom of the pan to release any browned bits. Stir in the ketchup and the remaining onion soup mix. Bring to a simmer.

Pour the gravy over the patties in the slow cooker.

Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours.

In a small bowl, whisk together the cornstarch and cold water to make a slurry. Pour the slurry into the slow cooker and stir gently. Cover and cook on HIGH for another 20-30 minutes, or until the gravy has thickened.

Serve hot over mashed potatoes or egg noodles, garnished with fresh parsley if desired.