Cut tuna into medium sized pieces, and place in a bowl
Chop onions, garlic and green chillies and add to bowl
Add turmeric, salt, pepper, and chillie powder to the tuna, onion, garlic, chillie mix
Mix tamarind concentrate in 1/2 cup of water, and mix well with other ingredients
Add mixture to a pan, and cook covered on low heat for 30 mins
Open cover and cook on medium heat for 5-10 mins until the curry thickens
Serve with white rice, bread or naan!