6 cups Fish stock
2 oz clarified butter
2 oz all-purpose flour
Heat the fish stock to a simmer in a medium saucepan.
Meanwhile, in a separate heavy-bottomed saucepan, melt the clarified butter over a medium heat until it becomes frothy.
With a wooden spoon, stir the flour into the melted butter a little bit at a time, until it is fully incorporated into the butter, giving you a pale-yellow-colored paste. This paste is called a roux. Heat the roux for another minute or so to cook off the taste of raw flour.
Using a wire whisk, slowly add the hot fish stock to the roux, whisking vigorously to make sure it's free of lumps.
Simmer for about 30 minutes or until the total volume has reduced by about one-third, stirring frequently. Use a ladle to skim off any impurities that rise to the surface.
The resulting sauce should be smooth and velvety. If it's too thick, whisk in a bit more hot stock until it's just thick enough to coat the back of a spoon.
Remove the sauce from the heat. For an extra smooth consistency, carefully pour the sauce through a wire mesh strainer lined with a piece of cheesecloth.
Keep the velouté covered until you're ready to use it.