Oysters on the half shell are baked with spinach, and a rich, buttery sauce flavored with champagne and shallots.
Pour salt into a rimmed baking sheet. Nestle oyster shells in the salt; set aside.
Bring cream to a boil in a saucepan. Reduce to simmer; cook until reduced by half. Remove from heat; set aside.
Meanwhile, place champagne, shallots, and vinegar in another small saucepan. Bring to a simmer, and cook until the liquid is almost evaporated. Add the reduced cream, and cook for 1 minute.
Remove pan from heat, and whisk in the butter, 1 piece at a time, adding each new piece before the previous one has melted completely. (The sauce should not get hot enough to liquefy. It should be the consistency of thin hollandaise.) Add 1/2 cup oyster liquor to the sauce, and season with salt and white pepper. Keep warm.
If using fresh spinach, rinse it thoroughly several times, and place in a saucepan with just the water that clings to the leaves. Cover the pan, and cook spinach until bright green and just tender, about 2 minutes. Drain well, and roughly chop. (Squeeze excess moisture from thawed frozen spinach).
Preheat broiler with rack 5 inches from the heat source. Divide spinach equally among the shells. Top with a shucked oyster. Spoon on enough sauce to cover the oyster, about 1 tablespoon per shell. Place under the broiler until sauce bubbles and edges of oysters start to curl, about 1 minute. Garnish with chives, and serve.