Step 1Toast the coconut in a dry skillet until crisp and golden brown, about 10 minutes for dried coconut and 15 minutes for fresh. Cool for 5 minutes, then grind finely in a food processor. Set aside.
Step 2Remove the outer layers of 4 lemon grass stalks; slice the stalks. Place the sliced lemon grass, onions, ginger, garlic and red pepper flakes in a food processor and process to a paste, adding a little water if needed.
Step 3Place the cubed beef, spice paste, coconut cream, water, salt and sugar in a pan or large wok. Bring to a boil, then reduce the heat to low and simmer, stirring occasionally, until most of the liquid has evaporated, about 1 1/2 hours. The gravy should be quite thick, with a film of oil on the surface.
Step 4Add the toasted coconut, makrut lime leaves, lime juice and, if desired, an additional stalk of lemon grass (outer layer removed and stalk bruised). Continue cooking over low heat for another 20 to 30 minutes, stirring frequently to keep the sauce from scorching. Remove from heat and taste for seasonings.
Step 5Serve with rice. Garnish with julienned cucumber and spicy sambal (Indonesian chile paste), if desired.