Well start by making a blond roux - Add the 3 tablespoons to a sauce pan over low heat.
Once the butter has melted, slowly whisk in the 3 tablespoons of flour.
Continue to stir or lightly whisk the mixture until it becomes pasty and golden in color.
Raise the heat to medium-low and slowly add in the stock, while whisking to ensure it doesn't separate.
Continue to stir the mixture until it becomes a creamy texture. You can add extra stock if you would like the sauce to be thinner in texture or less if you would like it thicker.
Recipe by Tasty Raisin at http://www.tastyraisin.com/2014/06/14/tomato-pesto-sea-bass-3/