Cut 3 romaine hearts in half lengthwise. Brush the cut sides with olive oil and season with salt and pepper. Grill cut-side down over medium-high heat until well marked, 1 to 2 minutes, then flip and cook 1 more minute. Transfer to a cutting board and chop. Toss with Greek salad dressing, chopped tomatoes, sliced red onion and cucumber, kalamata olives and crumbled feta.