Wash and dry all vegetables thoroughly. Chop lettuce into bite-sized pieces, slice tomatoes and cucumber, and thinly slice red onion. Dice bell peppers into small squares.

In a large mixing bowl, gently combine all chopped vegetables. If using feta cheese, crumble it over the top.

In a small bowl, whisk together olive oil, lemon juice, salt, and pepper to taste.

Drizzle dressing over the salad, and toss gently until all ingredients are coated.

Serve immediately or chill in the refrigerator for 30 minutes before serving.