01 -
Combine ground beef, finely chopped onion, egg, milk, Worcestershire sauce, Dijon mustard, salt, and black pepper in a large bowl. Mix just until uniform, avoiding overmixing.
02 -
Form the meat mixture into 4 oval patties approximately 3/4 inch thick and set aside.
03 -
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Sear patties for about 4 minutes on each side until browned, then transfer to a plate.
04 -
Add the remaining tablespoon of olive oil to the skillet. Sauté sliced mushrooms over medium heat for 5 minutes until deep golden.
05 -
Stir in minced garlic and cook for 1 minute until aromatic.
06 -
Pour in beef broth, gently scraping up browned residue with a wooden spoon. Bring to a simmer.
07 -
Slowly stir cornstarch mixture into simmering broth and mushrooms. Stir constantly until gravy thickens, about 2 to 3 minutes.
08 -
Stir in butter until melted and gravy is glossy.
09 -
Nestle browned patties into the skillet, spooning mushroom gravy over the top.
10 -
Reduce heat to low, cover, and gently simmer for 10 minutes to allow flavors to meld and patties to cook through.