Chop the kemiri nuts small and roast them golden brown in a dry pan. Immediately deposit on a cold plate to prevent sifting.

Halve and peel the lomboks and cut them small. Peel and chop the garlic and the shallots. Peel and chop the bun and the ginger. Halve and peel the tomatoes and cut them small.

Puff the coriander seeds in a dry pan and rub the seeds, along with the above ingredients, the salt and the trassi in a mortar into a coarse spice mixture. You can also use a food processor.

Bruise and knot the sereh and crush the salam leaf. Heat the coconut oil in a thick-bottomed pan and let it draw both spices on medium heat so that they give off all flavor. Add the coarse spice mixture together with the asem and the water after a few minutes. Let the whole thing slowly become hot. Simmer the bumbu 15 to 45 minutes until the water evaporates and it turns golden brown. Remove the sereh and the salam leaf. Allow to cool before use.

The bumbu can be kept in the fridge for up to two weeks and can be stored in the freezer for up to 6 months.