Preheat oven to 350° and grease 9×13" baking sheet with cooking-spray. Put one unrolled may of crescents on baking-sheet & then pinch together seams.
Caramelize-onions: In a large-skillet over moderate heat, melt 2 tbsp butter. Add onion and 2 sprigs-thyme & season with pepper and salt. Cook, stirring occasionally, until onions start to soften and turn slightly gold. Keep on cooking, stirring occasionally, until onions are tender & caramelized, 20-25minutes.
Construct squares: Spread an even layer of mustard onto ready crescent, leaving a ½" boundary clear on all borders. Top with a layer of roast beef & also a layer of provolone. Top with caramelized-onions, then unroll remaining tube of crescent-rolls & place on top of onion layer. Pinch together seams to seal.
Melt 2 tbsp of butter & brush over top of dough, then sprinkle with salt and parsley. Bake until dough is golden and cooked through, about 35-45minutes. (If dough is browning too fast, cover with foil.) Let cool 15minutes before cutting to squares.
Melt remaining 1 tbsp butter in a medium-skillet over moderate heat. Stir in garlic & cook until aromatic. Add beef-stock, Worcestershire-sauce & remaining two sprigs thyme, stripped out of stem. Season with pepper and salt. Simmer for 10minutes until reduced slightly.
Serve squares together with au jus on the side for dipping.