Begin by peeling and cutting the avocado into medium sized chunks. Place in a small bowl with the lime juice, salt, pepper and a pinch of cayenne. Stir to combine, gently breaking up the avocado but still leaving it chunky.
In a separate small bowl combine everything for the mustard mayo. Whisk together.
To assemble the sandwich, liberally slather one side of each slice of bread with the mustard mayo sauce. Lay the slices flat, mustard mayo side facing up. Place two slices of cheese on one slice of bread, followed by the avocado. Next add the sliced onions followed by the greens. Top with the other slice of bread.
Cut in half and serve.
Recipe Courtesy of Megan Mitchell