Mix mayo, Dijon mustard, honey, and apple cider vinegar in a small bowl. Season with salt.
Melt butter in a large skillet over medium-high heat. Toast bread slices until golden brown on both sides.
Spread honey mustard sauce generously on each bread slice.
Layer provolone cheese on one slice, followed by turkey, tomatoes, and avocado.
Season tomatoes and avocado with salt and pepper.
Top with mixed greens and close the sandwich.
Cut diagonally with a serrated knife.