Vegetarian Ramen

Trim dark green parts from scallions and thinly slice; set aside for serving. Coarsely chop white and pale green parts. Heat remaining 2 Tbsp oil in reserved pot over medium-high. Cook chopped scallions and ginger, stirring often, until scallions are charred in spots, about 4 minutes. Add tomato paste and cook, stirring occasionally, until it begins to stick to the bottom of pot and darkens slightly, about 2 minutes. Add mushrooms and kombu, then stir in 5 cups cold water. Bring to a boil, then remove from heat and let sit until mushrooms soften, about 10 minutes. Remove and discard kombu.