Heat a large pot over medium high heat. Add the sliced beef and cook until browned on both sides. Remove the beef and set it aside.
Add the onion, garlic, and ginger to the same pot. Cook for two minutes until fragrant. Stir in the red curry paste and let it cook for one minute.
Pour in the coconut milk and beef broth. Add the fish sauce and brown sugar. Bring the mixture to a simmer.
Add the rice noodles and red bell pepper. Stir gently so the noodles soften in the sauce. Cook for five to seven minutes until the noodles are tender.
Return the beef to the pot. Stir in the lime juice and let everything warm through for two minutes.
Serve in bowls topped with fresh cilantro.