In a large pot, heat the vegetable oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and grated ginger, cooking for an additional minute until fragrant.

Add the sliced beef to the pot and cook until browned, about 5 minutes.

Pour in the beef broth, coconut milk, peanut butter, soy sauce, fish sauce, and brown sugar. Stir well to combine and bring to a simmer.

While the soup is simmering, cook the rice noodles according to package instructions. Drain and set aside.

Add the sliced red bell pepper and bean sprouts to the soup, cooking for another 3-4 minutes until the vegetables are tender but still crisp.

To serve, place a portion of the cooked rice noodles in a bowl, ladle the hot soup over the noodles, and top with chopped green onions and fresh cilantro. Serve with lime wedges on the side.