In a bowl, mix coconut milk, tamarind concentrate, and curry paste; stir in cilantro.
Rinse tuna, pat dry, and turn over in coconut milk sauce. Lift out fish; save the sauce.
Pour sauce into a 1- to 1 1/2-quart pan; bring to a boil over high heat, stirring. Or bring to a boil in a microwave-safe container in a microwave oven at full power (100%). Keep warm.
Lay tuna on a lightly oiled barbecue grill over a solid bed of very hot coals or high heat on a gas grill (you can hold your hand at grill level only 1 to 2 seconds), close lid on gas grill. Cook tuna until bottom is pale color about 1/4 inch into fish (cut to test; center is red), about 2 minutes. Turn tuna with a wide spatula and cook until side on grill is the same color as it is on the top, and center is still pink to red, about 1 minute longer.
Stir cashews into rice, spoon onto plates, and top with equal portions tuna. Sprinkle with basil. Serve with warm coconut-curry sauce and add salt to taste.