Heat the olive oil in a large frypan over medium heat, add the onions and cook, stirring occasionally, until caramelized to a deep golden brown, about 20 min.
Meanwhile, make an au jus for dipping the sandwiches. Mix the onion soup mix (disregard package instructions) and broth in a saucepan. Bring just to a boil, stirring occasionally, then reduce heat and simmer uncovered 10 min. Turn off heat. Cover and keep warm. Preheat broiler with the rack in lower-third of oven.
Slice open demi baguettes lengthwise, without cutting all the way through, to open like books. Place, cut-sides up, on foil-lined baking sheet. Spread cut-sides with garlic-parsley butter. Spread mustard on 1 cut-side of each baguette. Finish by sprinkling parmesan over all cut-sides. Broil until the cheese is lightly golden and baguettes are toasted, 2 to 3 min. Set aside.
Roughly chop the roast beef and Montreal smoked meat into 1 in. (2.5 cm) wide pieces. Add to the caramelized onions in frypan, along with the thyme. Sauté mixture until heated through, about 5 min. Turn off heat. Leaving the hot mixture in the pan, fan provolone slices over top; let sit just until the cheese melts. Top the toasted baguettes with the pan mixture. Sprinkle with fried crispy onions. Close sandwiches and serve with a side of au jus for dipping.
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