Combine the ground beef, panko breadcrumbs, beaten egg, ketchup, Dijon mustard, dried oregano, and kosher salt in a large bowl, leaving the olive oil out for the skillet. The mixture should look evenly blended and slightly pasty, but stop mixing as soon as it holds together so the patties stay tender instead of dense.

Divide the meat mixture into 4 equal portions and shape each one into an oval patty about 3/4-inch thick. Keep the thickness even so the steaks finish at the same pace and do not crack at the edges.

Heat the olive oil in a large nonstick skillet over medium-high heat, then lay in the patties and sear for about 3 minutes per side before moving them to a plate. Wait for a steady dark-golden crust and a slightly quieter sizzle before flipping, which tells you the first side has released and browned properly.

Lower the heat to medium, melt the butter in the same skillet, and whisk in the flour until you have a smooth paste with no dry spots. When the roux smells lightly nutty and turns pale golden, it is ready to thicken the gravy without tasting raw.

Reduce the heat to medium-low and slowly whisk in the beef stock, then add the ketchup, Worcestershire sauce, and onion powder until fully combined. Pouring the stock in gradually keeps lumps from forming and helps the sauce turn smooth and glossy.

Stir in the sliced cremini mushrooms and let the gravy simmer for about 5 minutes, then season with salt and pepper to taste. The sauce should thicken enough to coat a spoon while the mushrooms soften but still keep their shape.

Return the patties and any juices to the skillet, nestle them into the gravy, cover, and cook for about 10 minutes more until the beef is cooked through and reaches 160 °F on an instant-read thermometer. The patties should feel firm but still springy, and the gravy will look darker and richer after the final simmer.

Spoon the steaks over mashed potatoes and finish with plenty of mushroom gravy from the skillet. The sauce should fall in a glossy ribbon, not a thin splash.