Warm the butter in the bottom of a Dutch oven set over medium-low heat. When the butter releases its foam, toss in the sliced leek, sprinkle it with salt and pepper, and then cover the pot. Let the leek sweat in the butter until tender, about eight minutes.
Uncover the pot, pour in the fish stock and drop in the potatoes. Turn up the heat to medium high and simmer the potatoes in the stock until tender, about twenty minutes. Drop the salmon into the stock, cover the pot, and allow it to continue simmering until the fish is opaque, three to five minutes. Remove from the heat, stir in the cream and dill. Salt the chowder as it suits you, and then ladle into soup bowls and top each bowl with a dollop of salmon roe.
If you can't find small potatoes, simply chop any peeled potato into 1-inch pieces.