Gruyere or Swiss are preferred. Trim off root ends and most of green part of leeks. Reserve white part plus a small part of green. Split in half and rinse thoroughly between leaves. Cut leeks lengthwise into thin strips and then crosswise into very small cubes. There should be about 5 cups. Peel potatoes and cut them lengthwise in half. Cut each half crosswise into very thin slices. There should be about 4 cups. Heat butter in a large saucepan and add chopped leek. Cook, stirring often, about 5 minutes. Add potatoes and water and bring to boil. Salt and pepper to taste. Simmer 20 minutes. Drain in colander, reserving liquid and solids. Heat oil and butter in heavy skillet and add bread cubes. Cook, stirring, until cubes are golden brown. Drain in sieve. Process leek and potato solids thoroughly while adding small amounts of reserved cooking liquid. Add only enough liquid to make a fine puree. Combine puree with remaining liquid in saucepan and bring to simmer. Add cream and salt and pepper to taste. Add nutmeg and heat thoroughly. Add oysters with their liquor and cook briefly, just until oysters curl. Serve sprinkled with parsley, with croutons and cheese on the side.