Grate about a quarter of the onion into a mixing bowl using the large holes of a box grater — this adds moisture and flavor without chunks. Set the remaining onion aside. Add the Worcestershire, ketchup, brown sugar, egg, and generous pinches of salt and pepper to the bowl. Whisk everything together until well combined.
Add the ground beef and panko to the bowl. Using your hands or a fork, gently combine everything just until the mixture holds together — overmixing will make tough patties. Shape into 8 oval patties about 1 inch thick, handling them as little as possible.
Transfer the patties to a plate and pop them in the freezer for 5-10 minutes. This firms them up so they won't fall apart when you sear them — a small step that makes a big difference.
⏱ 10 min
While the patties chill, dice the remaining onion into small pieces. Clean the mushrooms with a damp paper towel and slice them about ¼ inch thick — they'll shrink considerably as they cook.
Heat a large skillet over medium heat and add enough olive oil to lightly coat the bottom. Remove the patties from the freezer — they should feel noticeably firmer.
Sear the patties until they develop a nice brown crust, about 2-3 minutes per side. Don't move them too early or they'll stick. Transfer to a clean plate — they won't be fully cooked yet.
⏱ 6 min
Add the diced onion and sliced mushrooms to the same skillet without cleaning it — those browned bits are pure flavor. Season with salt and bump the heat up to medium-high.
Cook the vegetables, stirring occasionally, until they're deeply browned and the mushrooms have released their moisture, about 8-10 minutes. If the pan looks dry, add a splash more oil. You want real caramelization here.
⏱ 10 min
Sprinkle the flour over the vegetables and stir constantly for about 30 seconds. This cooks out the raw flour taste and helps thicken the gravy that's coming.
⏱ 30 sec
Pour in the beef stock and milk, stirring to scrape up any browned bits from the bottom of the pan. Season with salt and pepper, then bring to a simmer over medium-high heat.
Taste the gravy and adjust the flavor as needed — a splash more Worcestershire for depth, a touch of ketchup for sweetness, or a pinch of brown sugar to balance any bitterness from the mushrooms.
Nestle the seared steaks back into the gravy, reduce the heat to maintain a gentle simmer, and cook until the gravy thickens enough to coat the back of a spoon, about 4-5 minutes. Spoon the gravy over the steaks once or twice during cooking.
⏱ 5 min
Serve immediately while the gravy is still silky and the steaks are hot. Scatter chopped chives on top if you want a pop of color and mild onion flavor.