To fill a 1 liter terrine mold for about 10 appetizers:
Preheat oven to 325 degrees. Bring 2 qt. salted water to a boil. Blanch potatoes in boiling water until cooked and tender, cool in ice water. Blanch leeks for one minute, cool in ice water. Drain vegetables well and mix together with chopped herbs. Blanch chard leaves until tender, remove to ice water and then dry on towels. Refrigerate the chard until needed later in the recipe. Place cod in a food processor and puree , scraping down sides of bowl frequently. Add egg white and process briefly. Remove to a bowl and stir in cream, salt and cayenne. Press through a fine sieve. Chill in an ice bath. Stir in potato and leek mixture and smoked cod. Line terrine mold with plastic wrap allowing excess plastic to hang over edges and fill with cod mixture. Cover with overhanging plastic. Place terrine mold in a pan and add boiling water to come halfway up sides of terrine. Cover entire pan with foil and place in oven. Cook for 45 minutes. Check terrine with an instant read thermometer. Remove once middle of terrine reaches 135 degrees. Cool the pan it was baked in with the water for one hour at room temperature. Remove the terrine from the water bath and cool in the refrigerator overnight. Bloom gelatin sheets in a bowl of ice water. Squeeze out gelatin and add to vegetable stock. Bring to simmer to melt the gelatin. Remove terrine from mold. Place fresh plastic wrap into mold. Brush the inside of the mold with gelatin mixture. Line the mold with chard leaves then brush the leaves with gelatin mixture. Place unwrapped terrine inside of chard leaves. Brush top of terrine with gelatin, wrap overhanging leaves over terrine and brush again with gelatin. Wrap in plastic and refrigerate for one hour or more. Remove terrine from mold, slice and serve.