Spread butter, or brush with olive oil, one side of each bread slice.
Spread Caramelized Onion Mustard on the other side of bread.
Evenly distribute the pastrami and Swiss cheese over the mustard side of four slices of bread. Top with remaining slices of bread, mustard side down.
Heat a large skillet over medium heat. Place sandwiches in hot pan and cook several minutes per side until sandwich is golden brown on both sides and the cheese is melted. Serve immediately.