Shrimp cooking: to a large stock pot, add the water, kosher salt, and sugar, and bring to a rolling boil.
Once the water is vigorously boiling, remove the pot from the heat and add the shrimp. Set a timer for 2 and a half minutes and get a knife and a cutting board ready. After 2.5 minutes, remove one shrimp and cut it in half, and check to see if it's opaque all the way through. If it is, the shrimp is fully cooked, and you can remove the rest. If it's still translucent in the very center, add the shrimp back to the water and cook for an additional 30 seconds or so.
Remove the shrimp to a paper towel-lined plate or sheetpan and immediately place in the refrigerator for at least 30 minutes. You can leave this in the fridge overnight if you want to make the sandwiches the next day.
Shrimp dressing: when it's time to make sandwiches, chop each shrimp into 6 to 8 small pieces.
To a medium-sized bowl, add mayonnaise, Dijon mustard, salt, black pepper, lime juice, and zest, and mix to thoroughly combine. Add all the cut-up shrimp to the bowl with the dressing and carefully stir to coat fully.
Sandwich assembly: Toast the slices of bread on one or both sides. I like to do this in a pan with butter or bacon fat, but you can use a toaster if you'd prefer.
Spread mayonnaise and/or mustard on one slice of bread. Top with lettuce, bacon, and then the tomato slice. Lightly salt the tomato and pepper it if you'd like.
Top the tomato slices with a couple of spoons of dressed shrimp. Cover the shrimp with the second slice of toast.
Serve and enjoy.