Preparation
1.To poach the shrimp: Place 3 cups water in a medium saucepan. Squeeze the juice of both lemon halves into the water; add the squeezed halves to the water, and add the bay leaf and peppercorns. Bring to a boil over high heat and boil for 2 minutes. Remove the pan from the heat and add the shrimp. Cover and let stand off the heat for 8 minutes. Have ready a medium bowl filled with ice water.
2. Drain the shrimp into a colander, discarding the lemon halves, bay leaves, and peppercorns. Immediately transfer the shrimp to the ice water to stop the cooking and chill, about 3 minutes. Drain the shrimp and transfer them to a large dry bowl; refrigerate until needed. (The shrimp can be refrigerated for up to 2 days.)
3. To prepare the vinaigrette: Place the reserved grapefruit juice in a 1/4 cup measure. If necessary, add enough water to equal 1/4 cup. Puree the juice, 1/4 of the avocado, the lime juice, honey, ginger, salt, and pepper together in a blender until smooth.
4. To serve, remove the bowl of chilled shrimp from the refrigerator and add the grapefruit, mint, snow peas, and remaining 3/4 of the avocado. Pour the dressing over the mixture and toss gently to coat. Arrange the lettuce leaves on four plates and top with the shrimp mixture. Drizzle each salad with any dressing left in the bowl.