Set a rack in the middle of the oven and preheat the oven to 350°F. Line a rimmed baking sheet with parchment paper.
Arrange the tofu in a single layer on the baking sheet and bake for 20 minutes, flipping once halfway through. Remove from the oven and set aside.
Meanwhile, bring a large pot of water to a boil. Cook the noodles according to the directions on the package. Drain and rinse under cold water and set aside.
Whisk together the miso paste and tamari sauce and set aside.
In a large pot, heat the olive and sesame oils over medium heat. Add the onions, garlic, and ginger and cook for 5 minutes. Add 1 cup of vegetable broth and stir for 2 minutes. Add the mushrooms, white scallion slices, and carrots and cook for 3 minutes. Stir in the remaining 5 cups of vegetable broth and 2 cups of water. Add 1/4 teaspoon of pepper, the miso and tamari mixture, and stir to incorporate.
Raise the heat to medium-high and bring to a boil, then reduce the heat to low. Add the bok choy, spinach, and tofu and simmer for 15 minutes.
Divide the noodles into individual serving bowls, top with the soup and vegetables and garnish with the green scallion slices. Pass the sriracha separately.