Any oysters that don't open during cooking should be removed from the saucepan and opened with an oyster knife. As long as they were tightly closed before cooking, they will be fine to eat.
In a large saucepan, combine 1/2 cup of the wine with the water and bring to a boil over high heat. Using tongs, add the oysters to the saucepan in a single layer, cover and steam until they start to open, about 3 minutes. Using tongs, transfer the oysters to a plate as they open. Reserve the cooking liquid; you should have about 1/2 cup. Remove the oysters from their shells, put them in a bowl and cover with a damp paper towel. Reserve 12 concave shells.
Melt 1 tablespoon of the butter in a medium saucepan. Add the leeks, crumble in the saffron and stir. Cook over low heat until the leeks soften, about 8 minutes. Slowly add the reserved oyster liquid, stopping before you reach the grit at the bottom, and cook over moderate heat until the liquid is almost evaporated, about 3 minutes. Add the heavy cream and the remaining 2 tablespoons of white wine and simmer until the sauce is thickened, about 3 minutes. Season with salt and pepper.
Preheat the broiler. Arrange the 12 oyster shells on a rimmed baking sheet or in a shallow baking dish. Using a fork, divide the sautéed leeks among the shells. Set an oyster on the leeks in each shell and stir any accumulated oyster juices into the sauce.
Melt the remaining 1 tablespoon of butter. Whisk the egg yolk and the melted butter into the cream sauce and spoon the sauce over the oysters. Broil as close to the heat as possible for about 45 seconds, just until the sauce is bubbling and the oysters are lightly glazed. Serve at once.
Make Ahead: The recipe can be prepared through Step 3 up to I hour ahead; let stand at room temperature.