In a large bowl, whisk the egg and Worcestershire sauce first. Add the panko and spices, then gently fold in the ground beef. Do not over-mix; keeping the meat "loose" ensures the steaks stay juicy. Shape into 4 oval patties.
Heat a cast-iron skillet over medium-high heat. Sear the patties for about 3 minutes per side until a dark, caramelized crust forms. They don't need to be cooked through yet. Remove and set aside.
In the same pan, melt the butter. Add the onions and mushrooms. Cook for 8–10 minutes, stirring occasionally, until the onions are soft and a deep amber color. This caramelization is the secret to a dark, rich gravy.
Sprinkle the flour over the vegetables and stir for 60 seconds to "toast" the flour. Slowly pour in the beef bone broth while whisking. Stir in the Dijon mustard.
Place the seared steaks back into the skillet, spooning the onions and gravy over the top. Lower the heat, cover, and simmer for 10 minutes. The steam trapped under the lid finishes the steaks and allows the panko to soak up the savory sauce.
Serve these hot over a bed of Mashed Potatoes or thick egg noodles to catch all the extra gravy.