1. Heat a broiler to high heat. Prepare a baking pan with rock salt to level the oysters after shucking and for roasting.
2. Shuck the oysters and loosen the meat in the shell. Drain the liquor and reserve for another use. Arrange in the prepared pan to keep level.
3. Heat butter in a small sauté pan over medium heat. Sweat onions, enoki mushroom, and spinach until soft, then set aside to cool.
4. In a mixing bowl, combine mayonnaise, miso paste, mirin, and sweet soy.
5. Add the onion/mushroom/spinach mixture over the oysters.
6. Spread the mayonnaise mixture on the top of the veggies, sprinkle cayenne pepper and return to the pan.
7. Broil until golden brown, 2 minutes. Garnish with fish roe and serve immediately.