Peel cucumbers and halve them lengthwise. Seed one cucumber half, cut into large chunks and puree in a food process or blender until smooth. Add the yogurt, mint, and lemon juice and process until well combined. Set dressing aside.
Cut the remaining cucumber halves lengthwise into quarters, then cut crosswise into 1/4 inch thick slices. Cut the tomatoes into large chunks. Toss the romaine, cucumber, tomatoes, scallions, feta, and dressing together in a large bowl. Garnish each serving with an olive.