Toasted Steak Sandwich with Caramelized Onions and Brie

Method

1. Take the steak out of the fridge and place it on a plate. Pat it dry with paper towels and season generously with salt and pepper on both sides. Let it sit at room temperature while you caramelize the onions.
2. Heat 1 tablespoon of the olive oil in a large skillet over medium-high heat. Add the sliced onions and 1/4 teaspoon of salt. Cook until they start to soften and brown, about 5-8 minutes. Lower the heat to low and continue cooking, stirring occasionally, until the onions are very soft and caramelized, about 15 more minutes.
3. Remove from heat and stir in the Dijon mustard to coat the onions evenly. Transfer the onions to a plate or bowl, then wipe the skillet clean with a paper towel.
4. Preheat the oven to 350°F. Heat the remaining 1 1/2 tablespoons of olive oil in the same skillet over medium-high heat. When the oil is very hot and almost smoking, add the seasoned steak. Sear for 1 minute per side, then reduce the heat to low and cook for 7-10 minutes, turning once, until the center is very rare and an instant-read thermometer reads 125°F.
5. Transfer the steak to a plate and cover with foil. Let it rest for 10 minutes, then slice it against the grain into strips.
6. Spread some of the melted butter on the bottom of a 9x13-inch baking dish, just enough to coat. Lay the bottom halves of the buns in a single layer in the dish (if using a baguette, split it lengthwise into top and bottom, then cut crosswise to make two shorter sandwiches that fit).
7. Top the bread with the arugula.
8. Layer the steak slices over the arugula.
9. Add slices of Brie cheese on top.
10. Finish with the Dijon onions.
11. Spread the inside of the top buns with fig jam, then place them on top to close the sandwiches. Brush the tops with the remaining melted butter and cover the dish with foil.
12. Bake for 15 minutes, until the cheese is melted. Serve warm.