To make the spice paste, put all the ingredients into a mini food processor with 2 tablespoons water and blend until smooth.
Heat a wok over a high heat and add the oil. Once smoking, add the onion and stir-fry for 3–4 minutes, or until golden. Tip in the mustard seeds and stir-fry for 30 seconds, or until they start popping in the oil. Spoon in the spice paste