Peel 1 each medium parsnip and medium onion, wash, dice, and sauté in a pan with 1 tbsp of butter and same quantity of flour until brown. Add 2 cups of fish stock to the pan, season with salt and cook for 7-10 minutes. Take sauce off the heat and incorporate 1 egg yolk mixed with 1 tbsp of butter. Mix sauce well and strain. Add 1-2 tbsp of dry white wine or lemon juice.