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Heat oven to 180C/160C fan/ gas 4. Pour the oil into a small saucepan over a medium heat, add the shallot with a pinch of salt and cook, stirring frequently, for 5 mins until softened and beginning to caramelise.
Tip in the wine, cook for a couple of mins until reduced by half, then pour in the stock. Leave to bubble for 15-20 mins until you have a thick gravy.
Put the baguette in the oven for 5 mins to warm up and crisp, then cut in half lengthways. Spread one half with mustard and top with the roast beef.
Add the parsley, capers and butter to the gravy, stirring until the butter has melted. Season to taste, then spoon the gravy over the other half of the baguette, scattering over some spinach leaves. Halve, divide between two plates, then eat straight away, with more gravy on the side.