Arrange lettuce on 4 salad plates and set aside. In a frying pan, heat oil, then add garlic and onion. Cook until onions are translucent. Don't burn the garlic. Remove garlic. Add shrimp, salt and pepper and pan fry until shrimp are thoroughly pink. Then arrange 4 shrimp on each plate. Sprinkle each with a little lemon juice and serve warm.