Start by adding your onions to a small pan with 1 tbsp of butter. Saute over medium/high heat until onions are golden and translucent.
Place sliced pastrami on a microwave-safe plate and heat in 30-second increments until heated through.
Lightly butter the outsides of all four pieces of bread.
On the inside of the bread, build your sandwiches by spreading dijon mustard over each slice of bread, then layering 1 piece of cheese, hot pastrami, grilled onions, and one more slice of cheese.
OPTIONAL - if you want your toppings heated up, add those now too. (I prefer to add them a the end)
Place remaining 2 pieces of bread on top of the cheese, buttered side facing up.
Place two sandwiches on an indoor grill or in a frying pan and cook 2-3 minutes on each side, or until golden brown.
OPTIONAL - Open the sandwich up and top with your favorite toppings.
Serve immediately.
Notes
Nutritional values will vary based on your topping selections