Add sliced onions and all the ingredients: ginger-garlic paste, chilly powder, green chillies, coriander powder, cumin seed powder, vinegar, tamarind extract, sugar and required salt to the pork pieces and mix well in a heavy bottomed pan.
Cover and cook for 1/2 hour til pork is cooked or pressure cook it.
Since the pork it with the fat no need to add oil at all. Pork vindaloo in goan style is ready.