In a large pot, heat vegetable oil over medium heat. Add the sliced onion, garlic, and ginger, cooking until softened, about 3-4 minutes.

Stir in the beef slices and cook until browned, about 3-5 minutes.

Add the beef broth, coconut milk, peanut butter, soy sauce, fish sauce, brown sugar, and red curry paste to the pot. Bring to a simmer.

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

After the soup has simmered for about 10 minutes, add the sliced red bell pepper and bean sprouts. Cook for an additional 3-5 minutes until the vegetables are tender.

To serve, place a portion of 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.