Melt the butter in a large skillet over medium-low heat. Add the sliced onions and a pinch of salt; cook, stirring occasionally, for 18–20 minutes until deeply golden and jammy.
Add the garlic and cook 30 seconds until fragrant. Pour in the beef broth and Worcestershire sauce, scraping up browned bits. Simmer 8–10 minutes until reduced by about a third. Remove half the onions and set aside for the filling.
Warm the tortillas briefly (10 seconds in the microwave, wrapped in a damp paper towel). Spread each with 1 teaspoon horseradish cream cheese, leaving a half-inch border. Lightly brush with 1–2 teaspoons of the au jus.
Layer roast beef, reserved caramelized onions, and 1–2 slices of provolone across the lower third of each tortilla.
Roll tightly from the filled edge, tucking as you go. Wrap each roll in plastic wrap and refrigerate at least 20 minutes.
Unwrap and sear the rolls, seam-side down first, in a dry skillet over medium heat for about 2 minutes per side, until golden and the cheese has melted.
Rest 2 minutes, then slice into 1-inch pinwheels with a serrated knife. Strain the remaining au jus into small ramekins and serve alongside the pinwheels immediately.