Preheat oven to 300º. Whisk miso, oil, and 2 Tbsp. hot water in a small bowl. Crush mushrooms and kombu with your hands over a rimmed baking sheet. Add onions, carrot, celery, and garlic. Drizzle miso mixture over vegetable mixture and toss to coat. Bake, tossing halfway through, until vegetables are slightly shriveled golden and fragrant, 60–75 minutes.