Mix a couple of tablespoons of flour with enough water to make a creamy liquid. Season fishstock with whatever you want thesauce to taste of, bring to the boil and drizzle in flour mixture while stirring vigorously. Use a little at a time, simmering between additions to thicken, adding more as necessary to achieve the consistency you want. Then allow to simmer gently for another 15mins or more, stirring frequently.
Best stock to use is the cooking juices from the fish, especially if there were bones. Or you cOuld use a stock cube plus extra seasoning, replacing some of the water with white wine, for flavour, or Oatly/other dairy sustitute, for creaminess. A little olive oil also gives extra creaminess and flavour.