This is a ridiculously rich recipe, you can cut it up in bite size pieces to make apprizers. It is so good and evil at the same time.
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 for 10 min until lightly golden.
Caramelize onions: Melt 2 tbsp butter in a large skillet over medium heat. Stir in onion and 2 sprigs thyme and season with salt and pepper. Cook, stirring occasionally, until the onions begin to soften and turn slightly golden. Continue cooking, stirring occasionally, until the onions are soft and caramelized, 20 to 25 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, then unroll remaining tube of crescent dough and place on top of onion layer. Pinch together seams to seal. Spray with olive oil.
Bake until dough is golden and cooked through, about 15 – 25 minutes. (If dough is browning too quickly, cover with foil.) Let cool 5 minutes before slicing into squares.
Serves 2 – 4.