Step 1 Grate 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 2 Add 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.