How to Make It
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.