You can use "instant dashi" (like bullion cubes) or real fish to make the fish broth if preferred.
Garlic (sliced into very thin ovals) is also very good in this soup. Add with the scallion and tofu.
For a vegan version, ensure your dashi is mushroom-based (kombu/shiitake) rather than fish-based.
Do not boil the soup after adding miso, as high heat can destroy the beneficial enzymes and delicate flavor of the paste.