How to Make It
Rinse the onion and scallions in a fine sieve and pat dry; transfer to a bowl. Add the chives, lime juice, sesame oil and the 1 tablespoon of olive oil and toss.
Roll out the caviar between two sheets of plastic wrap 1/8 inch thick. Transfer the caviar to a cutting board and cut into 1/8-inch dice. Add the caviar to the bowl. Fold in the fish and season with salt and pepper. Spoon the tartare onto plates (using a 3-inch ring mold as a guide, if desired). Drizzle with olive oil, garnish with the tomatoes and herb sprigs and serve.