Mix all the ingredients together well. If you want, add more salt and lemon. It keeps in the fridge for up to 10 days.
FOR THE CRABCAKES:
1.About an hour before you start to cook, whisk together the egg, mayo, Old Bay, tarragon and salt in a small bowl.
2.In a medium bowl, use your hands to gently toss together the crackers and crab.
3.Fold in the mayo mixture with a delicate touch to keep those lovely crab lumps intact the best you can.
4.Fridge it for at least an hour (so the saltines turn into food glue) or up to a day.
5.Separate the crab mixture into four blobs, then lightly pack them into 1-inch-or-so-thick patties that are about as wide as the English muffins and put them on a plate.
6.Get a large heavy skillet hot over high heat, then add the oil and give it a swirl. When it smokes, add the patties with a little space between each one.
7.Lower the heat to medium and cook until these babies are pancake brown on both sides and hot through, about 4 minutes per side.
FOR THE MUFFS:
Mix the buttermilk dressing and tarragon in a little bowl and let it hang out.
Put the skillet back over medium heat.
Generously butter the cut sides of the English muffins, then cook them in batches, cut side down, until they're nice and toasty, 2 to 3 minutes. Set them aside.
Squeeze the whole lemon all over the crabcakes, then make sandwiches on the English muffins with the crabcakes, tarragon sauce and shredded lettuce.
Eat with the hot sauce handy in case anyone's feeling spicy.
Notes
The keys are really nice lump crabmeat, keeping those lumps intact by using a cherub's touch when you mix the ingredients and giving the mix some time in the fridge, so the saltines have a chance to bind it all together.