1. Bring 4 cups water, the ginger and kombu to boil in large, wide pot. Reduce heat and simmer, covered, 4 minutes. Turn off heat, sprinkle in bonito flakes and let sit 3 min. Strain broth through a cheese cloth lined fine mesh strainer into a bowl.
2. Rinse pot, then pour in broth. Add leek and carrts. Bring to simmer and cook 2 min. Add cod and scallops, simmer 1 minute. Add shrimp and cook 2 min more.
3. Whisk miso with lager in a small bowl. Remove stew from heat and gently stir in miso mixture. Serve with lemon zest and shiso for topping (optional).