There are few, if any, meals that cannot be transformed into a pizza. Case in point: the classic French Dip.
Let's start with the Cheese Sauce: Whisk together flour and 1/2 cup milk in a bowl until smooth. Heat remaining 1 1/2 cups of milk in a saucepan over medium-high until just simmering. Whisk in flour mixture you just made, and cook while constantly whisking. Keep going until it's thickened, about 3 minutes. Remove from heat; whisk in cheeses, salt, and garlic powder. Let it cool to room temperature, about 15 minutes.
Next, the Au Jus: Heat the stock and bouillon in a small saucepan over high. Bring to a boil, and cook until reduced to 1 cup, around 5 to 6 minutes. Whisk in the soup mix, and boil until it turns smooth while whisking often, about 1 minute. Let it sit for 5 minutes. Strain and discard any solids.
Now for the important part, the Pizza: Preheat oven to 500°F; place a pizza stone in oven to get things preheated. Heat 1 1/2 tablespoons of the oil in a large skillet over high. Add the onion and cook, stirring occasionally until lightly browned, about 2 minutes. Lower the heat to medium, then sprinkle onions with salt, and cook, stirring often, until browned and softened, 8 to 10 minutes. Remove from heat and cool slightly.
Roll the dough out on a lightly floured surface into a 14-inch circle. Sprinkle cornmeal on the pizza stone you heated up. Place rolled dough on stone. Work quickly, now! Spread 3/4 cup of the Cheese Sauce from step 1 on the dough leaving a 1-inch border. Top with roast beef and fold the slices slightly to create height. Top with onions, and arrange provolone slices in a circle pattern.
Bake the whole thing in your preheated oven until cheese is bubbly and browned, about 9 to 10 minutes. Brush edges of dough with remaining 1 tablespoon oil. Sprinkle pizza with pepper, and if desired, parsley. Cut into 8 slices, and serve with Au Jus and remaining Cheese Sauce.