In a small bowl, combine the mayonnaise, lemon juice and 1 1/2 tablespoons mustard. Season with salt; refrigerate.
Preheat a grill or grill pan to high. Brush the skirt steak with olive oil, season with salt and pepper and grill, turning once, for about 8 minutes for medium-rare; let sit. Thinly slice against the grain.
Meanwhile, preheat a waffle iron to medium-high. In a large bowl, stir the bacon and the remaining 1 1/2 tablespoons mustard into the prepared pancake batter.
Grease the waffle iron with cooking spray and, working in batches, pour in enough batter to barely fill each square; close and cook according to manufacturers instructions until golden. Spread each waffle with some of the lemon mayonnaise. Top with a lettuce leaf, tomato slices, steak slices and another waffle bread slice, mayo side down.