How to Make It
Prepare the Cheese Sauce: Whisk together flour and 1/2 cup of the milk in a bowl until smooth. Heat remaining 1 1/2 cups milk in a saucepan over medium-high until just simmering. Whisk in flour mixture, and cook, whisking constantly, until thickened, about 3 minutes. Remove from heat; whisk in cheeses, salt, and garlic powder. Keep warm.
Prepare the Au Jus: Heat stock and bouillon in a small saucepan over high. Bring to a boil, and cook until reduced to 1 cup, 5 to 6 minutes. Whisk in soup mix, and boil, whisking often, until smooth, about 1 minute. Let stand 5 minutes. Strain; discarding any solids.
Prepare the Fries: Heat 1 1/2 tablespoons of the olive oil in a large skillet over high. Add onion; cook, stirring occasionally, until lightly browned, about 2 minutes. Lower heat to medium, sprinkle with 1/2 teaspoon of the salt, and cook, stirring often, until onions are softened, 8 to 10 minutes. Transfer onions to a plate.
Heat vegetable oil in a large Dutch oven to 375°F. Cook half of the fries until golden brown for 4 to 5 minutes. Transfer to a plate lined with paper towels to drain. Sprinkle with 1/2 teaspoon of the salt. Repeat with remaining fries.
Heat remaining 1 tablespoon olive oil in skillet over medium-high. Add roast beef; cook, stirring often, until browned and slightly crispy, 30 to 45 seconds.
Place fries on a serving platter. Top evenly with onions, roast beef, and 1 1/2 cups of the Cheese Sauce; drizzle with half of the Au Jus. Sprinkle with parsley and pepper. Serve with remaining Cheese Sauce and Au Jus on the side.