Stir in 3 tablespoons of the butter and the pepper.

Fold in all but 2 tablespoons of the melted butter.

Cook the onion in 2 tablespoons of olive oil until soft, about 10 minutes.

Beat in two tablespoons of the olive oil and set aside.

Remove from heat and whisk in 2 tablespoons of the mustard.

When onions begin to dry out, stir in 2 tablespoons of the fish stock.

Beat the egg and stir in three tablespoons of milk.

Stir in two tablespoons of butter and allow to cool to room temperature.

Whisk in the vinegar, oil and 3 tablespoons of water.

Remove from the heat and whisk in two tablespoons of butter.