Make the beef mixture. Combine ground beef (1lb), half a grated onion, breadcrumbs (½ cup), egg (1), garlic (2 minced), Dijon (2 tsp), Worcestershire (2 tsp), tomato paste (2 tsp), bone broth (1 tbsp), and salt & pepper. Gently mix until just combined. Divide the mixture into 5-6 equal portions and shape into oval patties (not circle, don't want them to be confused with burgers).
Sear the patties. Heat the olive oil in a large skillet over medium-high heat. Sear the patties for 2–3 minutes per side, until deeply browned. They do not need to be cooked through yet. Remove to a plate.
Brown the onions and mushrooms. Add the half chopped onion to the same skillet and cook for 3–4 minutes. Add the mushrooms and cook for 5–7 minutes, don't stir often, until their moisture has cooked off and they are deeply golden. Add 2 minced garlic cloves and cook for about 30 seconds.
Deglaze with red wine. Pour in the red wine (¼ c) and scrape up the browned bits from the bottom of the skillet. Let it simmer for 1–2 minutes, until slightly reduced.
Make the gravy. Add 2 tablespoons of butter. Once melted, sprinkle in the flour (3 tbsp) and cook for 1–2 minutes, stirring constantly. Gradually whisk in the beef bone broth (2 c). Stir in the Dijon (2 tsp), Worcestershire (2 tsp), and rosemary (2 tsp).
Finish the Salisbury steak. Return the patties and any juices to the skillet. Simmer gently for 5–7 minutes, or until the patties are cooked through and the gravy has thickened.
Finish the gravy. Stir in the apple cider vinegar, starting with ½ teaspoon and adding more if needed. Taste and adjust the salt and pepper.
