In a large pot, heat the vegetable oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for another minute until fragrant.
Add the red curry paste to the pot and stir well, cooking for 2 minutes to release the flavors.
Pour in the vegetable broth and coconut milk, then add the soy sauce, fish sauce (if using), and brown sugar. Bring the mixture to a gentle simmer.
Meanwhile, cook the rice noodles according to package instructions. Drain and set aside.
Add the sliced mushrooms and red bell pepper to the pot, simmering for about 5 minutes until they are tender.
Stir in the bean sprouts and spinach, cooking for an additional 2 minutes until the spinach wilts.
Divide the cooked rice noodles into bowls and ladle the hot soup over them.
Top each bowl with sliced green onions and fresh cilantro. Serve with lime wedges on the side for squeezing over the soup.