To make tomato water, pulse overripe tomato with coarse sea salt in a food processor. Suspend in cheesecloth and set in strainer overnight allowing liquid to drop into a large bowl. Pass liquid through a coffee filter to remove all solids and season with mirin and cayenne, coarse sea salt and freshly ground pepper.
Arrange one scallop and one piece of uni in each well-rinsed scallop shell. Pour tomato water over scallop and uni and drizzle several droplets of mustard oil over top. Garnish with crushed black mustard seed.
Place seaweed on bed of crushed ice on small serving plate. Arrange five scallop shells in a circle on seaweed bed and serve immediately.
Increase quantities to create additional servings.