Firstly make the reduction: roughly chop the shallot or onion and place in a small saucepan with the tarragon leaves, vinegar, peppercorns and a splash of water. Bring to the boil and simmer until the liquid has reduced to around a tablespoon full. This should only take a couple of minutes so be careful not to let it boil dry.
Strain the reduction into a heat proof bowl (I use a Pyrex bowl) annd allow it to cool, then whisk in the 3 egg yolks, and place the bowl over a saucepan of simmering water.
Continuously whisk the eggs while adding spoons of butter. Whisk each piece of butter until it has melted before adding the next piece of butter.
When the butter has all been whisked into the eggs, continue to whisk the mixture until it thickens slightly. When you move the whisk through the sauce, it should leave a trail which slowly disappears.
Add lemon juice, salt and pepper, and the chopped tarragon leaves. Stir and remove from heat. As the sauce cools it will thicken further.
Serve on the side of steak and chips, or with a piece of salmon.
Never look back, and refuse to buy inferior versions from the shops!