Step 3: Make the Onion Brown Gravy
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In the same skillet, reduce heat to medium and add butter.
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Stir in sliced onions and cook 5–7 minutes, scraping the bottom of the pan, until the onions are soft and lightly golden.
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Sprinkle flour over the onions and cook 1–2 minutes, stirring, to form a roux.
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Slowly pour in beef broth while whisking to remove lumps.
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Add Worcestershire and mustard, then bring to a gentle simmer; cook a few minutes until the gravy thickens.
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Taste and season with more salt and pepper if needed.
Step 4: Simmer the Salisbury Steak in Gravy
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Nestle the browned patties back into the skillet, spooning some gravy and onions over the top of each one.
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Cover and simmer on low heat for 10–15 minutes, or until the steaks are cooked through (internal temperature about 160°F / 71°C) and the gravy is rich and glossy.
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If the gravy thickens too much, loosen it with a splash of broth or water.
