In a heavy bottom sauce pan, add coconut milk and orange juice, and over medium-high heat, reduce liquid by half. Thicken by mixing the cornstarch and water together to form a slurry and adding to the reduced mixture, stirring constantly. Reduce the heat and simmer for 2 to 3 minutes more. Add tarragon and orange zest, turn off heat, and allow to rest.
Take the tuna and season the whole outside with salt and pepper (if the tuna loin is too big, you may cut in half or quarters lengthwise for ease of cooking). Using a heavy bottom skillet or saute pan, add grape seed oil and heat until smoking. Place tuna loin in oil gently and leave for 2 minutes or until golden brown on that side. Turn and repeat on all sides of tuna. Remove from heat and allow to rest for 5 minutes.
To serve, place a little sauce on plate, slice the tuna thinly, and arrange on sauce. The tuna should have an appearance of being cooked on the outside and raw on the inside.