In a large mixing bowl, gently combine the ground beef, panko breadcrumbs, beaten egg, 2 tbsp ketchup, 1 tbsp Worcestershire sauce, onion powder, garlic powder, salt, and pepper. Be careful not to overmix.

Divide the mixture into 6 equal portions and shape them into oval patties, about 3/4-inch thick.

Heat the olive oil in a large skillet over medium-high heat. Carefully place the patties in the hot skillet and sear for 2-3 minutes per side, until nicely browned. You may need to do this in batches. The patties will not be cooked through. Transfer the browned patties to the bottom of your slow cooker, overlapping slightly if necessary.

Reduce the skillet heat to medium. Add the sliced onions to the same skillet, scraping up any browned bits from the bottom. Cook for 5-6 minutes, until they begin to soften and turn translucent.

Add the sliced mushrooms and minced garlic to the skillet. Cook for another 4-5 minutes, until the mushrooms have released their liquid and started to brown.

Whisk in the beef broth, 2 tbsp ketchup, 1 tbsp Worcestershire sauce, and Dijon mustard. Bring the mixture to a simmer.

Pour the hot gravy mixture over the steak patties in the slow cooker. Cover and cook on LOW for 4-5 hours or on HIGH for 2-3 hours, until the patties are cooked through and tender.

About 20 minutes before serving, prepare the cornstarch slurry. In a small bowl, whisk together the cornstarch and cold water until smooth. Pour the slurry into the slow cooker and gently stir it into the gravy.

Cover and cook on HIGH for another 15-20 minutes, or until the gravy has thickened. Garnish with fresh parsley before serving.