Heat oven to 400. Line a large sheet pan with parchment paper.
Melt butter in a skillet over medium heat and add the diced onion. Sprinkle with 1 teaspoon of Montreal Steak Seasoning. Cook for about 5 minutes until onions are translucent.
Roughly chop the roast beef slices into large bite-sized pieces. Add to the skillet with the onions and cook for about 3 minutes, until the roast beef just starts to brown.
Unroll pizza dough onto floured board or wax paper. Starting from the long side, cut dough into four even rectangles.
Cut each slice of American cheese and Provolone cheese in half. Place two pieces of American cheese on each rectangle, making sure to add toppings to only ONE half of rectangle (so that the other empty half will fold over toppings).
Spoon 1/2 cup of steak and onion mixture. Top with two pieces of Provolone cheese.
Fold dough over toppings and crimp edges with a fork.
Mix egg and water in a small bowl and brush over the top of each calzone. Sprinkle lightly with kosher salt and grated Parmesan cheese.
Transfer each calzone onto baking sheet and bake at 400 degrees for 15 minutes. Let calzones cool for 5 minutes before eating. Serve with Honey Dijon Sauce.
Mix all ingredients for Honey Dijon Sauce in a small bowl.