3 cups Dashi (dried fish and kelp stock)
1 tablespoon plus 1 teaspoon usukuchi (Japanese light-colored soy sauce), plus 1 tablespoon
2 teaspoons mirin (sweet rice wine)
½ teaspoon kosher salt
4 large eggs
8 thin, bite-size slices boneless skinless chicken thigh (from 1 small thigh)
One 3-ounce fillet delicate white-fleshed fish, such as fluke or flounder, cut into 8 thin bite-size slices
4 medium shrimp, peeled and deveined
8 drained canned gingko nuts or 4 jarred or vacuum-packed roasted chestnuts, halved
¼ cup loosely packed very roughly chopped mitsuba, thinly sliced scallions or thinly sliced shiitake mushrooms
Combine the dashi, 1 tablespoon plus 1 teaspoon of the usukuchi, the mirin, and salt in a small pot, bring to a boil, then let cool completely.
Gently mix the eggs in a medium bowl so they’re well mixed but don’t develop bubbles. Pour in the dashi mixture, stir well, then strain into a measuring cup, discarding any solids.
Briefly toss the chicken, fish, and shrimp in a bowl with the remaining 1 tablespoon soy sauce, shake to let any excess drip off, and divide them among the 4 small bowls or ramekins. Divide the gingko nuts among the bowls or ramekins, then divide the egg mixture evenly among them and cover with foil.
Line a wide, shallow-lidded pot or a Dutch oven with a paper towel. Add 1 inch of water and bring to a boil over high heat. Carefully add the bowls to the pot and cover with the lid. Cook until the custard turns pale, 3 to 4 minutes, then carefully remove the bowls from the pot. Add the mitsuba, scallion or shiitake mushroom to the surface of the custard and re-cover with the foil.
Reduce the heat to maintain a gentle simmer, return the bowls to the water, and cover the pot with the lid so it’s slightly ajar. Cook until the custard is just set but still jiggles when you shake the bowls (or a skewer inserted into the middle comes out clean), 12 to 15 minutes. Serve immediately.