In a medium bowl, combine the lemon zest and juice and garlic. Add the olive oil and cilantro, then whisk until it's all mixed together.
Place the shrimp in a bowl, pour in half of the lemon-cilantro dressing and stir to coat. Place in the fridge to marinate for 20 minutes or up to a couple hours.
Add the avocado to the bowl with the rest of the dressing.
Dice up the mango: Slice off both halves around the pit, score the flesh inside the skin and invert the skin to pop out the good stuff. Then, slice off the mango pieces so they fall into the bowl with the avocado. Add the red onion and some salt and pepper and gently mix it all together.
To serve, arrange 4 shrimp in the center of each lettuce leaf and spoon the mango-avocado salad on top. Sprinkle chives over the top to make 'em extra pretty.
To change things up: Make it a more "special" seafood dish by adding cooked lobster and crabmeat with the shrimp. Use jarred mango for softer, sweeter fruit. Serve the shrimp and mango-avocado salad over cold noodles for a shrimp pasta salad.