You can make this sauce at any time; it can be chilled overnight in the fridge which makes a pleasant contrast with the hot oysters and asparagus; or you can serve it warm or at room temperature.( We prefer it at room temperature.)
Begin by placing the egg yolks in a food processor or blender together with some salt & pepper ( taste to ensure the correct seasoning ) - switch on and blend thoroughly.
In a small glass bowl, heat the lemon juice and vinegar in the microwave until the mixture simmers, then switch the processor on again and pour the hot liquid SLOWLY onto the egg yolks in a steady stream.
Switch blender off, then in the same glass bowl, melt the butter in the microwave, but not too much - you don't want browned butter, just melted butter.
Once the butter is liquid and foaming, switch the processor on once more and pour the butter in - SLOWLY - in a thin steady stream, (too quickly and the sauce will split).Do this until the butter is all incorporated and the sauce has thickened.
Taste to check the seasoning - is there enough salt and pepper ? Enough vinegar and lemon juice? Add a little more of whichever you feel the sauce needs- to achieve the unique "tart" taste of a Hollandaise sauce -add a dash more lemon juice or white wine vinegar - if required. Remember - first taste before adjusting the final seasoning.