Peel and grate half the onion and finely dice the remaining half. Set aside.
In a medium bowl, gently mix to combine grated onion, panko, beef, egg, 2 tablespoons ketchup, 2 teaspoons Worcestershire sauce, 1 teaspoon Dijon mustard, ½ teaspoon kosher salt, and pepper using gloved hands.
Divide mixture into 5 sections. Transfer to a plate.
Shape each section into a 1-inch thick oval-shaped steak.
Heat a large nonstick skillet with olive oil over medium heat.
Add steaks and sear for 2 to 3 minutes until golden brown on the first side.
Flip and cook remaining side until golden brown. The steaks will not be cooked through.
Transfer steaks to a clean plate and set aside.
Add mushrooms, diced onion, butter, and remaining ¾ teaspoon salt to skillet.
Cook for 6 to 8 minutes, stirring occasionally, until mushrooms golden brown and tender.
Stir in garlic, if using, and flour. Cook for 1 minute.
Stir in half of the beef broth. Cook, stirring, to incorporate.
Stir in remaining broth, remaining 1 tablespoon ketchup, 1 teaspoon Dijon mustard, and 1 teaspoon Worcestershire sauce.
Add steaks back to the skillet.
Reduce heat to medium low and simmer, stirring liquid occasionally around steaks, until the gravy is thickened and the internal temperature of the steaks reaches 165 F.
Garnish with parsley and serve hot, ideally with mashed potatoes and your favorite veggie side.