Preheat the oven to 375 degrees F.
Blanch the head of cabbage in boiling salted water, about 5 minutes, then place in a basin of cold water. Remove the whole leaves and cut away the tough core. As you peel off the outer leaves, you may have to return the head of cabbage to the boiling water to soften the inner leaves. Dry on a clean towel and reserve.
Place the matzoh meal in a small bowl. Cover with 1 cup of stock and let soak until needed.
In a small skillet, heat the olive oil. Over medium heat, saute the onion until wilted, 4 to 5 minutes. Do not brown. Cool.
In a wooden bowl or on a chopping board, chop the fish fine with a chopper or large knife. Add the matzoh meal with the stock, the cooled onions, 3 egg yolks, the chopped parsley and tarragon, 2 teaspoons of salt, white pepper, and cayenne, and continue to chop until well combined. In a clean, medium bowl, whisk the egg white until firm but not stiff. Stir a little into the fish mixture, then, quickly but gently, and fold in the remaining whites. To test for flavor, bring a little fish stock to a simmer, add a small ball of the fish mixture and cook for about 5 minutes. Taste and correct seasoning.
Heat the remaining fish stock and spoon a little into an 11 by 17-inch baking pan. Divide the fish mixture into 12 portions, about 4 ounces each, and enclose each portion in 1 or 2 cabbage leaves. You will find that when the leaves get smaller, you will have to use 2 leaves to wrap the fish. As each package is formed, place in the prepared baking pan, seam-side down. This size pan holds the 12 packages comfortably. Pour the remaining stock over the fish and top with the julienned carrots and leeks. Cover the pan with foil and bake for 30 minutes. Let cool in the stock and refrigerate until needed.
Presentation: Place 1 package of fish on each of 12 plates, garnishing with some of the julienned carrots and leeks. Serve with homemade horseradish, white or red.*
To make white horseradish, finely grate peeled fresh horseradish into a small bowl, cover with plastic wrap, and refrigerate until needed.
To make red horseradish, boil 1/2 pound red beets until tender. Peel and then finely grate into a medium bowl. Add about 1/2 cup grated horseradish, or to taste, and combine thoroughly. Refrigerate, covered, until needed.
If you are concerned if the oils or other ingredients in these recipes are suitable for Passover, seek non-dairy substitutes or ingredients that are certified kosher for Passover.