This is a popular salad at some restaurants, with the tuna usually served pink.
Brush grill lightly with oil. Heat grill to medium-hot. Rub tuna with oil. Sprinkle with salt and pepper. Sear tuna until just pink in center, 3 to 3 1/2 minutes per side. Slice across into 1/4-inch strips.
Toss tuna, tomatoes, scallions, watercress, capers, olives, and anchovies with vinaigrette. Serve at room temperature.