Sprinkle tuna with pepper and coat with 1-2 T. olive oil. Cover with plastic wrap and refrigerate until ready to cook. Combine the remaining ingredients in a small bowl. Taste and correct the seasonings, adding salt and pepper to taste. Grill tuna approximately 4 minutes per side and serve with some of the vinaigrette spooned across each steak and garnish with mint sprigs. In place of the mint you may use a combination of mint and basil.