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Remove the saucepan from the heat, and gradually whisk in the flour and cream mixture to create a smooth soup. Add a splash of vinegar (about 1 tsp, you can always add more vinegar, but the soup will be hard to salvage if you add too much) and a good pinch of sugar. Whisk and taste; you’re looking for a good balance between sweet and sour, without masking the flavour from the fish stock. Add more of each seasoning as necessary. If you don’t have any vinegar to hand, I’ve found that using fresh lemon juice also works well, but be careful not to add too much and add an overly lemony flavour to the soup. This addition is not traditional, but again useful for using up fridge scraps