In a large salad bowl place lettuce, onion, olives and feta cheese; set aside. In a glass measuring cup, combine 2 tablespoons of the olive oil, the lemon juice, garlic, salt and pepper; set aside. Remove stems from mushroom caps; coarsely chop stems (makes about 3/4 cup). Cut caps in quarters, then into 1/4-inch thick slices (makes about 3 cups). In a large skillet, heat remaining 1 tablespoon oil; add chopped mushroom stems and sliced caps. Cook, stirring frequently until browned, about 4 minutes; cool slightly. Add mushrooms and reserved olive oil mixture to salad greens; toss gently. Serve immediately. Makes 2 servings.