To make the marinade, heat a wok or heavy-based frying pan and dry-roast the coriander and cumin seeds for 40–50 seconds until they release their fragrance. Using a spice grinder or pestle and mortar, grind together the coriander and cumin seeds, the coriander roots, garlic, peppercorns and salt. Turn this out into a bowl, add the sugar and soy sauce and mix to form a marin