Puree the chives and oil in a blender until smooth. Refrigerate in a squeeze bottle(can be made ahead and will keep for up to two weeks).
Peel the horseradish and coarsely grate. Turn 3/4 cup of grated horseradish into a flat dish and combine with the bread crumbs and herbs. In a second dish, whisk together the milk and eggs. Spread the flour, seasoned with salt and pepper, on waxed paper.
Only the top side of the fish is coated, not the skin side. Coat the top side of each fillet with the seasoned flour, shaking off the excess. Coat the same side with the egg and milk mixture and finally place it into the crumb mixture, pressing firmly to make sure the mixture adheres. Place fillets on a tray, cover with plastic wrap and refrigerate until cooking time.
Leek garnish: Cut the root end off the leeks and cut the white part into 3 1/2 inch lengths. Halve lengthwise and cut into fine julienne. Drop into cold water, leave a moment and lift out with your hands. Gently squeeze out excess water. In a skillet, heat two inches of peanut oil to 325°. Drop in the leeks, a handful at a time, and let them cook slowly until the bubbling stops (about 5 minutes). Remove with a slotted spoon and drain on paper towels. The leeks do not brown but remain light and crisp.
Mashed potatoes: Preheat the oven to 300°. Bring four quarts of water to boil in a large pot and add a tablespoon of Kosher salt. Add the potatoes and boil slowly until just tender. Drain thoroughly, transfer the potatoes to a baking pan and place, uncovered, in the oven to dry out (3-5 minutes). Puree the potatoes, season with salt and pepper and beat in the butter and milk to desired consistency. Keep warm.
Cooking the fish: Heat 1/2 inch of vegetable oil to medium high in a large skillet. Place the fish fillets crust side down in the hot oil and cook until golden brown, about 3 minutes. Turn and cook 3-5 minutes on the second side.
Presentation: Spoon a serving of mashed potatoes into the center of a plate. Place a fish fillet, crust side up on top. Drizzle chive oil around the potatoes and garnish with the crispy leeks. 6 servings.