Put the eggs and the mustard in a heatproof jug, and whisk.

Meantime, put the butter in a small pan and put on the heat.

Heat the butter til it foams and turns brown.

Once nut brown, take it off the heat and beating all the time, add bit by bit to your egg/mustard mixture. Again, if it splits, don't panic, keep beating, it'll come back.

Add lemon juice and seasoning to taste. If it's a little thick. add water to loosen it. The hot butter will have cooked the egg yolks, so there is not need to reheat this sauce.

Serve.