Heat skillet over medium flame.
Melt butter and add sliced onions.
Cook onions until soft and golden.
Add roast beef to warm through.
Sprinkle garlic powder and thyme.
Season with salt and pepper.
Remove mixture from the skillet.
Lay tortillas flat on a board.
Place cheese slice over each tortilla.
Add beef mixture evenly on top.
Roll tortillas tightly into logs.
Heat skillet again with butter.
Place roll ups seam-side down.
Cook until golden and crispy.
Flip to brown the other side.
In a saucepan, mix broth, Worcestershire, soy sauce.
Heat until lightly simmering for au jus.
Slice roll ups diagonally.
Serve warm with dipping sauce.