Filling: Heat the oil in a wok or frying pan and cook the onion over low heat until soft. Add the pepper-coriander paste, and cook, stirring, until the mixture is fragrant.
Add the pork, increase heat and fry, breaking up lumps to keep it crumbly. Cook until browned. Reduce heat to medium, stir in palm sugar and fish sauce. Cover and cook on low heat until pork is tender and liquid is absorbed. As it becomes drier stir frequently to ensure the mixture doesn't stick and burn. Remove from heat, mix in the chilli and crushed peanuts and cool to room temperature. Taste and add more salt or chilli. The filling must be strongly seasoned to stand up to the coating of bland sago.
To shape the balls: Using a wet spoon take equal amounts of sago and roll into balls with damp hands. Cover with a damp cloth. Wash and dry your hands and dust them with tapioca flour. Press thumb or finger into each ball to form a cup shape. Put in a teaspoon of pork mixture and seal the sago over the filling. Mould into balls again and cover with damp cloth until ready to cook. (If all are not required immediately, pack some in a freezer container and freeze, ready to steam when needed.)
Place the balls on oiled or non-stick paper strips in a steamer, leaving some space between as they increase slightly in size. Steam over fast-boiling water until the sago is quite clear, about 20 minutes. Remove the steamer from the heat and leave for 5 minutes for the balls to dry slightly before transferring to a serving dish. Sprinkle with crushed fried garlic or crushed roasted peanuts and garnish with chilli slices. Serve at room temperature.