Gather the ingredients. Begin by thinly slicing the 200 g Ripe Brie cheese.
Clean and slice the 250 g Cremini mushrooms.
Slice1 yellow onionthe 1 yellow onion.
The Vegetables
Melt 20 g unsalted butter in a large skillet over medium-high heat. Add the sliced 1 yellow onion and cook, stirring occasionally, until they are softened and translucent (about 5-7 minutes).
Add the sliced 250 g Cremini mushrooms to the pan with the onions and cook, stirring occasionally, until the mushrooms are browned and most of their liquid has evaporated (about 5-7 minutes). Season the onion and mushroom mixture with 1/4 tsp. salt and 1/4 tsp. freshly ground black pepper. Remove the cooked onion and mushroom mixture from the pan and set aside.Denis Tip: If you have a large amount of mushrooms and onions, you may need to cook them in two batches to avoid overcrowding the pan, which can lead to steaming rather than searing.
With the pan still hot, add the 30 ml white wine. Scrape up any browned bits from the bottom of the pan with a wooden spoon as these are packed with flavor. Let the wine bubble and reduce slightly for about 1-2 minutes. Add this reduced liquid back to the cooked mushroom and onion mixture.
The Steak and Brie
Add the remaining 20 g unsalted butter to the same skillet and increase the heat to high. Add the 250 g Ribeye steak and cook, stirring occasionally, until browned and cooked through (about 3-5 minutes, depending on thickness).
Season the steak with at least 1/4 tsp. salt and 1/4 tsp. freshly ground black pepper, or to taste, while it cooks. Once cooked, remove the steak from the pan.
After cooking the steak, let it rest for 5-10 minutes on a cutting board, loosely tented with foil. This allows the juices to redistribute throughout the meat, resulting in a more tender and juicy steak.
Thinly slice the rested steak into strips.
Preheat your oven to 350°F (175°C). Slice the 1 Baguette bread lengthwise or into individual sandwich portions. Place the baguette pieces on a baking sheet. The oven warming step is crucial for a good sandwich texture, ensuring a crispy exterior and warm interior. On one half of each baguette piece, arrange the 200 g Ripe Brie cheese slices.
Place the baking sheet with the baguette and brie in the preheated oven and warm until the baguette is crispy and warm and the brie is beautifully melted (about 5-8 minutes). Keep a close eye on it to prevent the bread or cheese from burning. Watch it carefully. Once the brie is melted, carefully remove the baguette from the oven.
Place the cooked steak strips on top of the melted brie.
Generously spread your sauce of choice (garlic Aioli, Dijon mustard or mayonnaise) on the other half of each baguette piece, the half without the cheese. Spoon the sautéed onion and mushroom mixture on top of the steak.
The Serving
Close the sandwich and serve immediately while everything is warm and the brie is still gooey. Enjoy your fresh Parisian Steak Baguette [Baguette de steak parisienne].