Blend the shallots, garlic, ginger, galangal, one lemongrass stalk, soaked chilies, turmeric, and coriander with a splash of water into a rough paste.
Toast the desiccated coconut in a dry wide pan over medium heat until deep golden, about 3 minutes, shaking constantly. Tip it out and set aside. This is kerisik and it is not optional; it gives rendang its signature nutty depth.
Heat the oil in the same pan over medium. Fry the spice paste for 5 to 6 minutes, stirring, until it darkens and the raw shallot smell is gone.
Add the beef and stir to coat. Pour in the coconut milk, then add the bruised lemongrass stalk, lime leaves, sugar, and salt.
Simmer uncovered over medium-low for 60 to 70 minutes, stirring every 10 minutes and more often near the end. A wide pan matters here; the liquid needs surface area to reduce. The curry will go from soupy to thick to nearly dry.
When the sauce has reduced to a dark paste clinging to the beef, stir in the toasted coconut and cook 5 more minutes, stirring so nothing catches. The fat will start to separate and the paste will fry slightly. That frying stage is what makes it rendang rather than curry.
Taste for salt. Serve with rice, or cool completely and portion into containers.