Rinse and dry the fish. Bring about 2 quarts (2 liters) water to a boil and drop in the potatoes. Drain them when they are half done, then peel and slice. Drain the anchovies, then chop together finely together with the garlic clove and parsley leaves. Thinly slice the onions.
Grease a baking pan with 1 tablespoon of the oil. Divide the potatoes in half and lay half of the slices in a single layer in the baking pan. Sprinkle with the onions and half the chopped anchovy mixture. Drizzle with 2 tablespoons of the oil and lay the fish slices on top, close together. Season with salt and pepper and arrange the remaining potato slices on top in a single layer. Spread the remaining anchovy mixture over the potatoes and drizzle with the remaining oil.
Bake in an oven preheated to 400°F (200°C) for about 20 minutes, or until lightly browned on top. Let rest for a few minutes before serving.