In a pot, heat a drizzle of oil over medium heat. Add the diced onion, garlic, and ginger, and sauté until fragrant and translucent, about 5 minutes.
Add the dried shiitake mushrooms to the pot, along with the vegetable broth. Bring to a simmer and allow the mushrooms to steep for about 15 minutes.
Remove the shiitake mushrooms and set aside. Stir in the soy milk, miso paste, and mirin. Mix until the miso is completely dissolved, and let the broth simmer for an additional 10 minutes.
In a separate pot, cook the ramen noodles according to package instructions. Drain and set aside.
Slice the reserved shiitake mushrooms and return them to the broth along with the baby spinach. Cook until the spinach wilts, about 2 minutes.
To serve, divide the cooked ramen noodles into bowls and ladle the hot broth over the noodles. Top with baked tofu (or eggs), garnishes like scallions, sesame seeds, and optional toppings such as nori, bamboo shoots, and bean sprouts.