Heat 1 tablespoon oil in a wok over high heat. Stir fry the beef in batches until browned. Transfer to a plate. Set aside.
Heat 1 tablespoon oil in a wok over medium high heat. Cook onion until soft, add garlic, lemongrass, star anise, and cinnamon stick. Stir fry for about 3 minutes or until fragrant.
Add coconut cream and tamarind paste and cook for about 3-5 minutes or until the sauce has thickened slightly. Optional: Add ½ tablespoon light brown sugar to add sweetness (or use more according to your liking)
Return the beef sirloin into the wok, add green beans and stir fry for about 3 minutes.
Remove from the heat and serve immediately with jasmine rice.