Grilled Beef and Onion Panini

3 sweet onions , thinly sliced 2 tablespoons honey 1 tablespoon Dijon mustard 1 tablespoon apple cider vinegar black pepper softened butter 8 slices rye bread[...]

3/4 cup thinly sliced red onion 3/4 cup thinly sliced white onion 2 tablespoons honey 1 tablespoon country-style dijon mustard 1 tablespoon cider vinegar land[...]