1. Cut the avocado in half, remove the pit and peel and slice it. Sprinkle the slices with 2 tablespoons of lemon juice and set aside.
2. In a large mixing bowl, whisk the remaining lemon juice, orange juice, shallots, tarragon, lemon zest, olive oil, salt and pepper together. Add the shrimp and refrigerate until ready to serve.
3. Arrange the greens on 4 large serving plates. Lift the shrimp out of the lemon vinaigrette and mound them in the center of the lettuce. Drizzle the lettuce with the remaining vinaigrette. Arrange the mango, pineapple and avocado around the shrimp.Serving Size: 1 salad