In a small saucepan, combine beef broth, Worcestershire sauce, garlic powder, and black pepper. Bring to a simmer over medium heat. Reduce heat to low, cover, and keep warm while preparing the roll-ups.

Melt butter in a non-stick skillet over medium-low heat. Add thinly sliced yellow onion and a pinch of salt. Cook slowly, stirring occasionally, for 10-15 minutes, or until deeply golden brown and softened. Set aside.

Lay out one large flour tortilla on a clean surface. Layer one-quarter of the roast beef evenly over the tortilla, leaving about a 1/2-inch border around the edges. Top the roast beef with two slices of provolone cheese, slightly overlapping. Spread one-quarter of the caramelized onions over the cheese. Starting from one edge, tightly roll up the tortilla. Repeat with the remaining tortillas and fillings.

Preheat your air fryer to 375°F (190°C). Brush the exterior of each rolled tortilla lightly with olive oil or melted butter. Using a sharp knife, slice each roll-up into 2-inch pieces (approximately 2 pieces per tortilla, total 8 pieces). You can also cook them whole and slice afterward if preferred, but slicing before helps them crisp evenly.

Place the sliced roll-ups in a single layer in the air fryer basket, ensuring not to overcrowd. You may need to cook in batches. Air fry for 6-8 minutes, flipping halfway through, until golden brown and the cheese is melted and bubbly.

Carefully remove the French Dip Tortilla Roll-Ups from the air fryer. Serve immediately with the warm au jus for dipping. Garnish with fresh parsley if desired.