Step 1Grate the peel from the lemon (you should have about 1 1/4 teaspoons) and set it aside. Cut the lemon in half and squeeze 2 tablespoons juice into a blender jar. Set aside the remaining half lemon.
Step 2Add the egg, salt, cayenne pepper and Dijon mustard to the blender jar. Blend to combine.
Step 3With the blender running, slowly add the oils in a thin stream until all the oil is incorporated and the mayonnaise is thick. Turn off the blender. Stir the reserved grated lemon peel into the mayonnaise.
Step 4In a small bowl, spoon one-fourth cup mayonnaise onto the crabmeat. Toss until the crab is coated with dressing.
Step 5Lightly spread each roll with mayonnaise. (You will have about half the mayonnaise, one-half cup, remaining. Save it for another use.) Spoon the crab salad onto the bottom halves of the rolls and squeeze a little juice from the remaining half lemon over each. Top the crab salad with watercress leaves and the upper halves of the rolls to make sandwiches.