Step 1

Place the egg yolks into a medium-sized glass bowl and half-fill a small saucepan with boiling water. Make sure the bowl containing the yolks will fit snugly inside the saucepan without touching the water, as this would cause the yolks to overheat.

Step 2

Melt the butter in a small saucepan, then reduce the heat down to its lowest setting to keep warm.

Step 3

Add the lemon juice and mustard to the egg yolks with a pinch of salt. Use a balloon whisk to mix it all together, then place the bowl over the saucepan of water. Whisk vigorously for 1 minute over a low heat so the eggs begin to cook and thicken slightly.

Step 4

Use a tablespoon measure to drizzle 1 tbsp of the warm butter into the egg yolks, whisking continuously. Continue to add the butter spoon by spoon, whisking well before each addition, until it is all incorporated. You should be left with a thick, silky smooth sauce (see my tips for how to rescue it if it starts to split).

Step 5

Add 1 tsp water to loosen the sauce and season to taste with extra salt and lemon juice. The sauce can be kept warm over the pan of warm water (off the heat) while you prepare the rest of your dish. Serve with poached eggs on toast, on al dente asparagus or with pan-fried white fish.