To accompany fish, asparagus, etc.

Chef's tips
If you don't have tarragon, you can use tarragon vinegar. 
The secret of making a successful bearnaise sauce is to use butter at room temperature, neither too cold nor melted.

1. reduce the vinegar with the shallots and garlic in a saucepan for 20 minutes over very low heat; strain the reduction;
2. place the egg yolks in the upper part of a double boiler; add the reduction; whisking constantly, add the butter piece by piece until thick and emulsified; season with salt and pepper;
3. remove from the heat and finish with a squeeze of lemon juice.