Heat oil in a 3-quart pot. Add onions, carrots, celery, shiitakes, garlic, kombu and salt. Sweat covered for approximately 10 minutes on low heat.
Add wakame, arame. Continue to sweat covered for another 10 minutes. While soup is cooking peel ginger with the back of a spoon. Grate into shreds and then using cheese cloth squeeze the juice out of the ginger and set it aside
Add water, bring to a boil. Reduce heat to low and simmer uncovered for another 10-15 minutes. Turn off heat. Let broth stand for 5-10 minutes. Add ginger and lemon juice.
Temper miso in bowl by mixing with 2 cups of broth. Add tempered miso back into the soup pot. Serve, garnished with scallions and nori.