Place a medium saucepan over a medium heat and add the unsalted butter. Let it melt and bubble gently.
Stir in the plain flour and whisk continuously for 1–2 minutes, until the mixture turns a pale golden brown and smells nutty.
Gradually whisk in the beef stock, a little at a time, to prevent any lumps from forming.
Bring to a gentle simmer over a medium heat, whisking occasionally, then simmer for 3–5 minutes, until the gravy has thickened to your liking.
Remove from the heat, season with the sea salt and freshly ground black pepper to taste, then serve the gravy piping hot.