Heat the vegetable oil in a large, deep skillet or pot over medium heat. Add the chopped onion, minced garlic, lemongrass, and galangal (or ginger). Sauté until the onions are soft and translucent.
In a small bowl, mix together the turmeric powder, coriander, cumin, and chili powder. Add this spice mixture to the skillet and cook for about a minute, stirring continuously.
Add the beef chunks to the skillet and brown them on all sides, ensuring they are evenly coated with the spice mixture.
Pour in the coconut milk, along with the cinnamon stick and kaffir lime leaves (if using). Stir well to combine all the ingredients.
Reduce the heat to low and let the beef simmer uncovered for about 2-3 hours, stirring occasionally, until the meat is tender and the sauce has thickened. If needed, you can partially cover the skillet during cooking to prevent excessive evaporation.
After a couple of hours, stir in the tamarind paste and season with salt to taste. Continue cooking for another 15-20 minutes to allow the flavors to blend.
Once the beef is tender and the sauce has thickened to your liking, remove the lemongrass, galangal (or ginger), cinnamon stick, and kaffir lime leaves (if used).
Serve the Beef Rendang hot with steamed rice, and garnish with freshly chopped cilantro.
