Preheat your oven to 425°F (220°C). Wash and peel the russet potatoes. Cut them into crinkle-cut fries using a crinkle cutter or a knife. Place the cut fries in a large bowl, toss with olive oil, chopped rosemary, and sea salt until well coated.

Spread the fries in a single layer on a baking sheet lined with parchment paper. Bake in the preheated oven for about 30-35 minutes, flipping halfway through, until they are golden brown and crispy.

While the fries are baking, season both sides of the filet mignon steaks generously with Montreal steak seasoning. Let them sit at room temperature for about 15 minutes.

In a large skillet, heat a tablespoon of olive oil over medium-high heat. Once hot, add the seasoned filet mignon steaks. Cook the steaks for about 4-5 minutes on each side for medium-rare, or until they reach your desired doneness. Adjust the cooking time based on the thickness of the steaks. Remove the steaks from the skillet and allow them to rest for 5 minutes before slicing.

In the same skillet, add butter and sliced onions. Sauté over medium heat until the onions are soft and caramelized, about 8-10 minutes. Sprinkle the flour over the onions and stir well, cooking for an additional minute.

Gradually add the beef broth and Worcestershire sauce, stirring constantly until the sauce thickens. Season with black pepper to taste.

Plate the filet mignon alongside the rosemary-salted crinkle fries. Drizzle the thick brown onion sauce over the steak. Enjoy your delicious Montreal-seasoned filet mignon with crispy fries and savory onion sauce!