Roughly chop 1 lb carrots , 12 oz celeriac, 1 small red onion, and smash 1 large garlic clove.
Place all ingredients into soup pot and add 4 cups water. Bring to boil, reduce heat and simmer, covered for approximately 15 minutes, until all veg are soft.
This step is optional, but I like to use a potato masher to smash the vegetables roughly, so that some pieces can be left to add texture to soup after pureeing.
Once you've smashed (or not) the vegetables, use an immersion blender (or food processor-see notes) to puree the soup to desired consistency. It is entirely up to you whether you prefer a velvety smooth texture or a smooth texture with some small pieces of vegetable to add texture.
Stir the 6 tbsp white miso, 1/4 cup mirin, and 2 tbsp toasted sesame oil into the pureed soup. Taste and adjust by adding more of any of these ingredients. See notes. If soup feels too thick for your liking, stir in a bit more water to loosen it up.
Serve with a tablespoon of the 4 tbsp furikake seasoning sprinkled over each of four servings. Depending on the exact size of your veg and whether you weigh them or not, you may find this makes more like 6 lunch size servings.