|Combine all the ingredients in a small bowl and mix with your fingers, gently bruising the tarragon leaves to release their aromatic oils. Cover and let sit for at least 20 minutes; use within a few hours. Sprinkle on tuna steaks to taste.
Fire up the grill, concentrating the coals beneath the spot where the steaks will go so as to maximize the heat. Or if you’re not quite ready to break out the grill yet, crank up the heat under a grill pan.
Lightly oil and season the tuna medallions. When the grill or grill pan is very hot, throw on the tuna. Now keep your mitts off until the sear is complete. This takes 1 to 2 minutes. When the time comes to flip, you’ll know the sear is done if the tuna moves easily and does not stick to the grates or pan. Cook the other side for another 1 to 2 minutes.
Make the salsa. Peel the oranges and cut them into segments; cut each segment into thirds and combine these with the onion, tarragon, and olive oil in a small bowl. Season with the salt and toss well.
Slice the tuna steaks across the grain. Spoon the salsa over the steaks, and pour a glass of chardonnay to go with the wine salt.