Miso Dashi Soup with Halibut

FOR THE DASHI: Bring water and kombu to rapid simmer in large saucepan over medium heat. Turn off heat and discard kombu, then stir in bonito flakes and let sit to infuse flavor, about 5 minutes. Strain dashi through fine-mesh strainer, pressing on solids to extract liquid, then return dashi to saucepan. (You should have about 5½ cups dashi. Cooled dashi can be refrigerated for up to 2 days.)

FOR THE SOUP: Heat oil in Dutch oven over medium heat until shimmering, then add scallion whites, ginger, and garlic and cook until fragrant, about 30 seconds. Add carrots and dashi to pot and bring to simmer.

Submerge halibut in dashi and return to simmer. Reduce heat to medium-low, cover, and gently simmer until fish flakes apart when gently prodded with paring knife and registers 130 degrees, 6 to 8 minutes. Transfer halibut to individual serving bowls.

Whisk miso into dashi in pot and bring to simmer. Add spinach and cook until wilted, 15 to 30 seconds. Remove from heat, then ladle soup over halibut in bowls. Sprinkle with scallion greens and sesame seeds and serve.