Separate leaves of lettuce for cups. Core tomato and cut into ¼" dice. Peel and mince shallot. Mince cilantro, stems and leaves. Quarter lime. Pat shrimp dry, and season with a pinch of pepper.
Make Pico de Gallo and Spicy Mayo
In a mixing bowl, combing tomatoes, shallot (to taste), half the cilantro (reserve remaining for garnish), squeeze juice from two lime quarters, ¼ tsp salt, and a pinch of pepper. In another mixing bowl, combine mayonnaise and Sriracha (to taste).
Make the Batter
In another mixing bowl, mix together tempura mix and 2½ Tbsp. water until the consistency of pancake batter. Add shrimp and toss or stir to coat completely.
Cook the Shrimp
Line a plate with paper towels. Place a medium non-stick pan over medium-high heat and add 3 Tbsp. olive oil. Add shrimp to hot pan, letting excess batter drip off before adding. Cook until browned and shrimp reaches a minimum internal temperature of 145 degrees, 2-4 minutes per side. Remove shrimp to towel-lined plate.
Finish the Dish
To lettuce cups, add shrimp, pico de gallo, remaining cilantro, spicy mayo and a squeeze of lime juice, if desired. Place three cups on a plate.