Step 1Heat the oven to 175 degrees. Refrigerate the oysters in their liquid and place the shells on a baking sheet. Bring a small pot of water to a boil and blanch the leeks until tender, 4 to 6 minutes. Drain the leeks and spread them out on a towel to cool.
Step 2In a medium saucepan set over medium heat, combine the shallots and 2 tablespoons Champagne, cooking until the liquid is reduced to almost nothing. Over very low heat, slowly whisk pieces of butter into the nearly dry pan, one at a time. The mixture should thicken as you whisk. Thin the sauce with the remaining one-half cup Champagne and taste for seasoning, adding a little salt if desired; the sauce should be only lightly salted (the oysters are already salty).
Step 3Place the oyster shells in the oven. In a small pot set over low heat, gently simmer the oysters in their liquid until they begin to curl up ever so slightly. Remove the shells from the oven and arrange them on a serving platter lined with rock salt. Fill each shell with a spoonful of cooked leeks and a warm oyster, and finish with the Champagne butter and a sprinkle of chives.