Preheat oven to 350°. Warm 2 tablespoons olive oil in a 4- to 6-quart pot over medium heat. Add onion and sea salt; sauté 5 minutes. Stir in garlic and sauté a minute more. Add mushrooms and stir until coated with oil. Add cauliflower and water, cover, bring to a boil, reduce heat, and simmer 15 minutes.
While soup cooks, toss sliced shiitakes with olive oil and sea salt. Spread mushrooms on a parchment-lined baking sheet. Roast 25 minutes. 3. In a blender, purée soup (in batches) until creamy. Transfer soup to a clean saucepan; place over medium heat. Add umeboshi vinegar and simmer 2 minutes. Season with salt to taste.
. Just before serving, toss roasted mushrooms with nori strips. To serve, ladle soup into bowls. Place small mounds of shiitake and nori garnish in the center of each bowl.