Wash the fish, dry with paper towel, drizzle with lemon juice, sprinkle with salt, set it aside . Drizzle fish with white wine, set 25 milliliters of fish broth aside. Pour remaining broth in the steamer cup, add clove, bay leaf, juniper berries, thyme, peppercorns and lemon zest, bring to a boil. Oil fish and place in the steamer, simmer for 25 minutes on low heat. Mix the remaining white dry wine with fish broth, milk and vegetable broth. Fry the flour in the butter and pour over broth-milk mixture, cook for 10 minutes until thickening, stirring occasionally. Chop tarragon and add in the sauce with sour cream and different mustard, season with sugar, salt and lemon juice. Serve the fillet with mustard-tarragon sauce.