Cook the shrimp: In a pot, combine 1 quart water with 1 tablespoon salt. Add peppercorns and bay leaves and bring to a boil over high heat.
Prepare an ice bath: Add enough ice to a large bowl until half full. Sprinkle the ice with 2 tablespoons salt, and top with enough cold water to cover. Set aside.
When the water boils, stir the shrimp into the hot water and turn off the heat. Let cook until just pink and opaque for 1-2 minutes, depending on size of shrimp. Drain shrimp and transfer to the ice bath. Stir well and set aside until shrimp are completely chilled, 3 to 5 minutes. Drain shrimp again, and transfer to towels to dry.
Make the dressing: In a medium bowl, stir together tomatoes, red onion, oil, fruit juices, herbs, and mustard—season to taste with salt and pepper. Set aside 1/2 of the dressing. In first half of the dressing, add shrimp and mango, toss to coat, and chill for at least 30 minutes (or up to 1 day).
When ready to serve, toss the lettuce with the 2nd half of the dressing. Peel and dice the avocado, and toss both into the shrimp salad. Serve chilled in a serving bowl or on individual plates lined with lettuce leaves. Enjoy!