In a medium bowl, combine onions, vinegar, water and salt and set aside for 30 minutes. In a small bowl, combine mayonnaise and mustard and stir to combine. To assemble sandwiches, spread 8 slices of bread with 1 tablespoon of mayo-mustard sauce. Top with 4 to 6 slices of turkey, 2 tablespoons of pickled onions and a small bunch of arugula. Top with remaining bread, half, and serve immediately.