Razor clams - also called bamboo clams - have sweet, tender meat. In this dish, they're grilled with butter and a little fish sauce, then topped with uni and dried fish roe, which changes the dish from homey to luxurious. It can get even more extravagant depending on how much of the uni you use, and the quality. One small box - of the type you can buy at supermarkets, not the size that sushi chefs use - is enough for this dish, but you can use more, if you like.
Karasumi is Japanese dried fish roe. Most of the fish roe you'll find at Japanese shops is actually from Taiwan (where it's called wuyuzi); you can also use Italian bottarga.
Run a sharp paring knife between the clam and the shell, to detach the meat. Cut the clams along the length and remove and discard the soft dark parts (the stomach). Briefly rinse the clam meat under cool, running water, then blot with paper towels. Break the shells in half at the hinge and select the prettier halves for serving. Dry the shells with paper towels, then place them cup side up on a pan that holds them in one layer. Lay the clam meat back in the shells (if you like, cut the clam meat into bite-size pieces).
Melt the butter. Very lightly brush the fish sauce over the clam meat, then drizzle with melted butter and sprinkle with freshly ground black pepper.
Heat the oven grill/broiler to high. When it’s very hot, slide the pan under the grill, so the clams are about 2½cm (1in) away from the heat element. Cook for about a minute, or until the clam meats starts to curl; do not overcook, or the clams will be tough.
Place the clams on two dinner plates, or four appetiser plates. Arrange the sea urchin (uni) pieces on the clams. Use a fine-tooth rasp-type grater to shave the dried fish roe (karasumi, wuyuzi, or bottarga) over the clams. Garnish with micro-greens and serve immediately.