In a bowl, combine ground beef, egg, breadcrumbs, minced onion, garlic, Worcestershire sauce, salt, and pepper. Mix gently until combined.
Divide mixture into 4-6 portions. Shape into oval patties with a thumb indentation in the center.
Heat oil in skillet over medium-high heat. Sear patties 3-4 minutes per side until golden. Remove and set aside.
Melt butter in same skillet. Add sliced onions and cook until caramelized (5-7 minutes).
Sprinkle flour over onions, stir 1 minute. Gradually whisk in beef broth and bring to simmer until thickened.
Return patties to skillet. Simmer in gravy 10 minutes until beef reaches 160°F (71°C).
Notes
Storage: Refrigerate for 3 days or freeze for 3 months. Reheat in skillet for best texture. Variations: Use ground turkey, gluten-free breadcrumbs, or serve over mashed potatoes.