The most involved aspect of the recipe for the roast is the Sri Lankan curry powder–which has all the same ingredients as normal curry powder (coriander seeds, cumin, fenugreek, tumeric, cinnamon, chiles…), except that you roast some of the spices first to release the oils. You also need curry leaves and tamarind.
Basically, start by soaking about a tbsp of tamarind in hot water.you fry some onions slowly, add some fenugreek, ground cumin and coriander and then add curry leaves and Sri Lankan curry powder along with some garlic and chiles. Set aside. Then you brown the meat in the same pan. Add a little more Sri Lankan curry powder. Place the onion mixture and the meat in a slow cooker. Add water and the tamarind pulp (with solids and pod material filtered out). Add 2 cups of coconut milk. Cook on high heat about 4 hours.