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To prepare the gratin, slowly melt the butter over a low heat and, in a separate pan, gently heat 300ml of the lobster bisque until warm. In a third pan heat water until simmering, placing a bowl with the egg yolk, water and lemon juice over the top. Whisk continuously until the egg mixture becomes thick and glossy. Remove the bowl from the heat and, still whisking continuously, gradually add first the melted butter followed by the bisque. Pass the finished gratin mix through a fine sieve