In a medium size skillet, over medium heat, place the pat of butter. Once melted add the thinly sliced onions. Stir and turn until onions are caramelized a nice golden brown. Set aside till ready to assemble.
In a small bowl mix the Mayo and Mustard together to form the mustard sauce.
Pre-heat the oven to broil.
In another medium size skillet warm the pastrami while the hot dogs are cooking on the grill and the buns are toasting.
When the buns are done, place one slice of cheese in the middle of the bun. Place the buns on a cookie sheet in the broiler and watch them carefully until the cheese is melted and bubbly.
Once done remove from oven and add a generous serving of pastrami. Then add the grilled hot dog, followed by the caramelized onions and mustard sauce. Serve with a dill pickle and Enjoy!