1½ cups thinly sliced yellow onion (about 1 medium onion)
1½ cups beef broth
2 tsp ketchup
2 tsp Worcestershire sauce
¾ tsp Dijon mustard
1½ tbsp cornstarch
1½ tbsp water
Mashed potatoes, to serve
Method
Combine beef, panko, egg, garlic powder, onion powder, paprika, 2 tablespoons of the parsley, 1 teaspoon of the salt, and ½ teaspoon of the pepper in a large bowl. Gently mix until all seasonings are evenly distributed; form into 4 (¾-inch) patties.
Heat oil in a large nonstick skillet over medium-high. Add patties in a single layer; cook until browned on both sides, 4 to 5 minutes per side and transfer to a plate.
Return skillet to heat, and add mushrooms and onion; cook, stirring occasionally, until softened, about 2 minutes. Transfer to a 6-quart slow cooker. Place patties in a single layer on top. Whisk together broth, ketchup, Worcestershire, and mustard. Pour over patties and vegetables.
Cover and cook on LOW 4 to 5 hours. After cooking, transfer patties to a platter; cover to keep warm.
Turn slow cooker to HIGH, and whisk together cornstarch and water; whisk into sauce mixture. Cook, uncovered, stirring often, until thickened, about 3 minutes. Stir in remaining 2 tablespoons parsley. Serve gravy over patties with mashed potatoes.