Prepare the vegetables. Wash and thoroughly dry the lettuce. Slice the cucumber and onion, and halve the tomatoes. For a milder onion flavor, soak the sliced onion in cold water for 5–10 minutes, then drain.
Make the dressing. Whisk together the olive oil, lemon juice, vinegar, oregano, garlic, Dijon and black pepper.
Assemble the salad. Arrange the lettuce in a large serving bowl. Add the cucumber, tomatoes, onion and olives, then scatter the feta over the top.
Dress and serve. Drizzle with the vinaigrette just before serving and toss gently. Taste before adding salt—the feta and olives contribute plenty of salty flavor.