Step 1

Combine dashi powder, 1 tablespoon mirin, and 2 cups warm water in a medium bowl. Stir to dissolve powder. Transfer 1 tablespoon dashi mixture to a small bowl. Add soy sauce, oil, and remaining 2 teaspoons mirin; set dashi sauce aside. Set remaining dashi mixture aside for custard.

Step 2

Set a large bamboo steamer over a large pot filled with 1-inch of simmering water. If using fresh peas, place in a small wide bowl and set bowl in steamer. Cover and cook until peas are tender, about 5 minutes (if using frozen peas, do not cook). Repeat with shrimp and mushrooms, placing each in separate small wide bowls, and cook in steamer until shrimp are just opaque and mushrooms are softened, about 2 minutes for each.

Step 3

Gently mix eggs in a large bowl to blend (do not create bubbles); stir in dashi mixture reserved for custard. Pour custard through a fine-mesh sieve into a large bowl or a 4-cup measuring cup.

Step 4

Toss peas, mushrooms, and scallions in a medium bowl and combine. Divide all but 2 tablespoons pea mixture among 6 small heatproof bowls or 8-ounce ramekins (set remaining pea mixture aside for serving); pour dashi custard over pea mixture, dividing evenly. Tightly cover each bowl with plastic wrap.

Step 5

Place bowls in steamer, cover, and cook until custard is just set but still jiggles slightly in the center, 10-15 minutes.

Step 6

Top custards with reserved dashi sauce, pea mixture, and shrimp.