TO MAKE SPICE PASTE, peel and roughly chop the onions, garlic, lemon grass, galangal, ginger and turmeric. Roughly chop the chilli peppers. Shred the lime leaves. Place all of these, plus the shrimp paste, in a food processor or blender (in batches) and process until you have a smooth paste. You can add a little water if necessary.
TO MAKE SAUCE, heat the oil in a saucepan