In a high sided saute pan or sauce pan with a lid over medium-high heat, add the sake and the mussels and cover with the lid. Steam the mussels until all of the shells pop open, about 2 to 3 minutes. Using a slotted spoon transfer the mussels to a bowl and discard the liquid. Detach the top shell of the mussels and discard. Transfer the mussels still attached to the bottom half of the shell to a baking sheet lined with coarse or rock salt.

Preheat the broiler in the oven and arrange the oven rack as closer to the broiler as possible.

In a small bowl combine the mayo, Sriracha, soy sauce, ponzu, mirin and tobiko and mix well. Spoon about 1 teaspoon of the sauce over each mussel.

Transfer the baking sheet to the oven and broil the mussels  until the sauce begins to bubble around the edges about 1 to 1 1/2 minutes. Remove from the oven and cool slightly. Garnish with the Cajun Caviar or black tobiko and green onions and serve immediately.