An intensely concentrated broth pairs with delicate white fish and rich shiitake mushrooms. Gently steaming the fish gives it light and flaky texture. The citrus zest added just before serving brightens the whole dish.
Combine dashi, mirin, and soy sauce in a medium saucepan, and cook over medium, stirring often, until warm, about 5 minutes. Reduce heat to low, cover, and keep warm.
Bring a large pot of water to a boil over medium-high. Divide fish among 4 (8-ounce) heatproof bowls; place bowls in a bamboo steamer basket. Ladle 1/4 cup warm dashi mixture into each bowl; add a small leaf of lemon verbena (or sprinkle evenly with chopped scallions). Place bamboo steamer basket over boiling water, cover, and steam 4 minutes.
While fish steams, stir together yam and shiitake slices in a small bowl. When fish has steamed 4 minutes, spoon 1 tablespoon yam mixture into each bowl. Cover and steam 2 minutes.
Sprinkle each fillet with a pinch of yuzu zest; serve with shredded nori.
Earthy Pinot Meunier–based Champagne.