1. In a food processor, pulse tomatoes with sea salt until chunky. Line a strainer or coffeemaker basket with a coffee filter and transfer tomatoes to the coffee filter. Set strainer over a large nonreactive bowl or coffeemaker carafe. Refrigerate this contraption at least 12 hours: the tomatoes’ intensely flavored, clear juices will filter into the bowl. To the tomato water, add mirin, cayenne pepper, and ground pepper.

2. Cover a dish with crushed ice. (If serving this as an appetizer, use dinner plates. A large serving tray or tiered platter would work well if these are to be served as hors d’oeuvres.) For an individual appetizer, arrange 5 shells in a ring on the ice. Place a scallop (or piece of scallop) and uni lobe side by side in each shell. Spoon seasoned tomato water into each shell to cover the shellfish, 1 ½ to 2 teaspoons per shell. Drizzle several drops of mustard oil over each, and garnish with a pinch of black mustard seed. Serve immediately.