Cook the ramen noodles according to package instructions. Drain and set aside.
In a large pot, heat the vegetable oil over medium heat. Add the sliced onion and sauté for about 3-4 minutes until translucent.
Stir in the minced garlic and sliced mushrooms. Cook for an additional 5-7 minutes until the mushrooms are tender.
Pour in the vegetable broth, soy sauce, miso paste, sesame oil, and grated ginger. Bring to a simmer and let cook for 5 minutes.
Add the baby spinach and cooked ramen noodles to the pot. Stir gently to combine and heat through for another 2-3 minutes.
Serve hot, garnished with sliced green onions, sesame seeds, and a soft-boiled egg if desired.