Then add the water and vinegar reduction and whisk.
Place the gastric, egg yolks, salt, and pepper in a bowl and whisk in a double boiler until fluffy and cooked through. When the egg mixture shimmers like a pudding, it's done.
Now whisk in a little melted butter and heat the pan gently in a bain-marie.
Continue whisking until the mixture thickens, then add the melted butter in a thin stream.
Continue whisking until the sauce is thick and glossy.
Season with salt and pepper if needed. Keep warm in a bain-marie or thermos.