In a large pot, heat the vegetable oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for another minute until fragrant.
Add the diced chicken breast to the pot, cooking until browned on all sides, about 5-7 minutes.
Pour in the chicken broth and water. Add soy sauce and miso paste, whisking to combine. Bring the mixture to a boil.
Once boiling, reduce the heat to a simmer. Stir in the coconut milk and allow the soup to simmer for 10 minutes.
Add the ramen noodles and cook according to package instructions, typically about 3-4 minutes.
In the last minute of cooking, add the fresh spinach to the soup. Season with salt and pepper to taste.
Serve the soup hot, garnished with chopped green onions and any additional toppings you desire.