Preheat a non-stick pan or panini style grill, over medium heat.
Spread the mustard on both slices of bread. Layer the bread with cheese, sauerkraut, pastrami, more cheese, and slice of bread.
If using a pan, add the oil, then the sandwich and cover with a lid. Cook on both sides until the bread is golden brown and the cheese is melted. If using a panini style grill, lightly spray with non-stick spray and cook the sandwich until it is golden brown and cheese is melted.