Put all the spices in a spice grinder or mortar and pestle, and grind to a powder. Sieve it and keep it in an airtight container. Try to use it fresh, or at least within 2–3 months.
Marinate the meat with one clove of garlic, 2 tbsp Colombo powder, salt and pepper, the lime juice and 1 tbsp oil for at least 2 hours, or preferably overnight.
Remove the meat from the marinade and pat dry with paper towels. Heat the remaining 2 tablespoons of oil in a large, heavy pot over a medium–high heat. Add the meat and cook, stirring occasionally until browned.
Add the onions, the remaining garlic, the spring onions, tamarind paste, the remaining Colombo powder and 4 tbsp of water, then the aubergine, sweet potato, yam, the whole chilli, thyme, parsley, salt and pepper. Reduce the heat, cover the pot and cook over a low heat for 45 minutes, until the meat is very tender. Serve hot, with boiled white basmati rice or coconut rice.