French Dip Squares

Preheat oven to 350° and grease a 9"-x-13" baking sheet with cooking spray. Place one unrolled can of crescents on prepared baking sheet and pinch together seams.  Bake until edges are slightly golden, about 12 minutes. Remove from oven and set aside. Meanwhile, caramelize onions: In a large skillet over medium heat, melt 2 tablespoons butter. Add onion and 2 sprigs thyme and season with salt and pepper. Cook, stirring occasionally, until onions begin to soften and turn slightly golden. Continue cooking, stirring occasionally, until onions are soft and caramelized, about 20 minutes. Remove from heat. Build squares: Spread an even layer of mustard onto prepared crescent, leaving a 1/2" border clear on all edges. Top with a layer of roast beef and a layer of provolone. Top with caramelized onions. Unroll remaining tube of crescent rolls and place on top of onion layer. Press seam into bottom crust to seal all edges. Melt 1 tablespoon butter and brush all over top of crescent dough, then sprinkle with parsley and salt.  Bake until dough is golden and cooked through, about 35 minutes. (If dough is browning too quickly, cover with foil.)  Let cool 15 minutes before slicing into squares. Meanwhile, make au jus: Melt remaining 1 tablespoon butter in a medium skillet over medium heat. Stir in garlic and cook until fragrant. Add beef stock, Worcestershire sauce, and remaining 2 sprigs thyme, stripped from stem. Season with salt and pepper. Simmer for 10 minutes until reduced slightly.  Serve squares with au jus on the side for dipping.