Start by making the bumbu spice paste. For this small quantity I find this easiest to do with a pestle and mortar. Start with the coriander seeds and peppercorns, then add all the other ingredients and grind to a paste.
Put the bumbu in a large frying pan with the lime leaves, lemongrass and galangal. Drizzle in the oil and stir-fry until fragrant. Loosen the paste with a ladleful of water.
Add the mushrooms and turn to coat in the spices. Add the coconut milk, palm sugar and a good pinch of salt. Cook for 5-10 minutes. The mushrooms will release liquid as they fry. Towards the end of cooking, stir through the kecap manis, sliced chillies and tomato. Taste for seasoning.