Make the balsamic reduction: Place 6 tbs balsamic vinegar, 1 med lemon juiced, and1 clove garlic, peeled and halved in a small saucepan over medium-low heat. Simmer until the mixture reduces by half. Turn off the heat and reserve until needed. This sauce does not have to be hot for serving. The reduction will thicken slightly as it cools. In a small bowl, combine 1 ½ tsp salt, 1 tsp coriander, 1 tsp paprika, and ¼ tsp cayenne pepper. Lay 4 tuna steaks out on a plate and sprinkle the spice mixture evenly on all sides. Evenly coat the tuna steaks with freshly ground black pepper, and gently press it in, so that it adheres to the surface, being careful not to smash the flesh. Place a thick-bottomed frying pan or cast iron skillet over medium-high heat. Add oil and swirl to coat the pan. When you see small wisps of smoke, add the tuna to the pan and sear the steaks for about one minute per side, or until the desired doneness is reached. Remove the tuna steak and place on a cutting board. For presentation, cut each steak diagonally into 4 to 5 slices and fan on a plate. Serve with a small amount of sauce drizzled alongside. Garnish with additional lemon if desired.