Cream and oyster liquor (the liquid that surrounds the mollusks in their shells) are a rich, flavorful base for this stew, which is enhanced with velvety mascarpone cheese and topped with salmon roe.
Melt butter in a large saucepan over medium heat. Add shallot; cook, stirring, 1 minute. Add leek; cook, stirring, until soft, about 2 minutes more. Stir in 1 teaspoon salt and 1/4 teaspoon pepper.
Add potato, and cook, stirring occasionally, 5 minutes. Stir in oyster liquor, half-and-half, and cream. Simmer until potatoes are almost tender, 8 to 10 minutes. Add oysters, and gently simmer until edges of oysters are ruffled and potatoes are tender, 3 to 5 minutes.
Remove from heat. Stir in mascarpone; season with salt and pepper. Divide among bowls. Top each with a dollop of creme fraiche and 1 teaspoon caviar.