Melt the unsalted butter in a medium saucepan over a moderate heat.
Sprinkle in the plain flour, whisking constantly for 1–2 minutes until it turns a pale golden colour.
Slowly pour in the beef stock in a thin stream, whisking continuously to incorporate it fully before adding more.
Bring to a gentle simmer over a moderate heat, whisking occasionally, and let it cook for 3–5 minutes, or until it thickens enough to coat the back of a spoon.
Remove the pan from the heat, stir in the sea salt and freshly ground black pepper, then taste and adjust the seasoning if needed.
Serve the brown gravy hot over mashed potatoes, roast meats, Yorkshire puddings or your favourite accompaniment.