In a bowl, combine the crackers with the sour cream, egg, harissa sauce, tomato paste, lime juice, parsley, and mustard. Add the crabmeat and stir to combine. Season with salt and pepper. With your hands, shape four patties.
In a large skillet, brown the patties in the oil for 2 to 3 minutes on each side. Toast the buns.
Generously spread homemade ketchup on the bottom buns and top with the patties. Add tomato slices and lettuce. Close with the top buns.