1In a bowl, combine ground beef, minced onion, panko breadcrumbs, egg, worcestershire sauce, minced garlic, salt, and pepper. Mix gently with your hands until just combined. Shape into four oval patties about 0.75 inch thick.
2Heat vegetable oil in a large skillet over medium-high heat. Once shimmering, place patties in the pan and sear 3–4 minutes per side until golden brown. Transfer to a plate.
3In the same skillet, add sliced mushrooms and sliced onion. Cook 5–6 minutes, stirring occasionally, until softened and lightly browned. Scrape up any browned bits from the bottom.
4Pour in beef broth and mustard, stirring to combine. Return patties to the skillet, nestling them into the gravy. Reduce heat to medium-low, cover, and simmer 12–15 minutes until the patties are cooked through.
5Stir in sour cream and thyme, heating gently without boiling. Season with salt and pepper to taste. Serve the patties with mushroom gravy spooned over the top.