Cook the ramen: Bring a small pot of water to a boil. Add the ramen, lower the heat to medium-high, and cook for about 4 minutes, or until soft and easily separated with a fork. Drain and rinse in a colander. Set aside.
Cook the onions: While the ramen is cooking, heat a Dutch oven or a large pot over medium-high heat. Add a splash of vegetable broth and the onion. Cook for 6 to 8 minutes, until softened, adding more broth as needed to prevent sticking.
Add veggies: Add the carrots, celery, and ginger. Cook for about 6 minutes, or until the vegetables start to soften.
Add aromatics: Stir in the garlic, Italian seasoning, pepper, and turmeric. Cook for 1 minute, until fragrant.
Simmer: Add the rest of the broth and bring to a boil. Reduce the heat to medium-low and simmer for about 15 minutes.
Add miso: Remove the pot from the heat. Ladle about 1 cup of the broth into a small bowl. Whisk in the miso paste until smooth, then stir the miso-broth mixture back into the soup.
Finish: Add the spinach and stir until just wilted. Add the cooked noodles, lemon juice (start with 1 tablespoon and adjust to taste), and basil (if using). Serve immediately.