Heat a gas grill to medium high or prepare a medium-hot charcoal fire. (If using charcoal, be sure the grate is hot, too.) Put the shrimp in a large bowl and mix with the olive oil until well coated. Put the shrimp on the grate directly over the heat and grill until pink and almost cooked through, 4 to 5 minutes, turning once halfway through. Let cool completely.

An hour before serving, whisk together the tequila, lime juice, orange juice, ketchup, and green Tabasco. Peel the shrimp, cut them into large pieces (about 1/2 inch), and toss with the tequila mixture. Cover and refrigerate for 1 hour. Just before serving, season the shrimp mixture with salt and pepper. (Alternatively, omit the kosher salt at this stage and sprinkle on a coarse sea salt like fleur de sel just before serving.) Gently fold in the tomatoes, avocados, and scallions, mixing well. Using a slotted spoon, portion the mixture into individual serving bowls or margarita glasses. Garnish with a sprinkling of onion (if using), a wedge of lime, and the optional sea salt. Serve immediately with the crackers.