Preheat the oven to 340 F. Cover the roasting dish with non stick foil. Put corned beef in and sprinkle top with packaged pickling spices. Wrap top tightly in foil and bake for 120 minutes or until easy to slice.
With a sharp knife, cut the corned beef into thin slices. Smear butter on one side of 2 slices of bread. Set one with butter down into a pan on stove over medium heat. Spread mustard on the non buttered side of the other piece of bread. (a slice of cheese like Swiss could be added on top of the mustard so it melts in the pan if you like too)
Place 4-5 slices of corned beef on bread in the pan with other mustard side facing down with buttered side facing up. Once browned on one side, flip to the other.
Remove, can add slices of cool pickles and/or lettuce leaves inside before serving.