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 ginger, cooking for an additional minute until fragrant.
Add the sliced beef to the pot and cook until browned, about 3-4 minutes. Season with salt and pepper.
Pour in the beef broth, soy sauce, peanut butter, and red curry paste. Stir well to combine and bring to a simmer. Let it cook for 10 minutes to allow the flavors to meld.
Meanwhile, 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, divide the cooked rice noodles among bowls. Ladle the hot soup over the noodles. Top with fresh cilantro, green onions, and a squeeze of lime juice.