• Caramelize the onions. Melt the butter in a skillet over medium-high heat. Add the thinly sliced onions to the skillet and cook, stirring, until they start to sweat. Then add sugar, salt, and pepper. Cook, stirring until the liquid has evaporated.
• Deglaze. Add 2 tablespoons of water to the onions and cook, scraping any browned bits from the bottom of the skillet. Repeat this process until the onions have that deep amber color. This process may take up to 30 minutes. Once done, set the onions aside.
• Spice up the mayo. Add the mayonnaise, yogurt, sriracha, smoked paprika, and lemon juice to a bowl. Stir to combine and set aside.
• Prepare the steak. Pat the steak dry, then season it to taste with salt and pepper. Heat the olive oil in a cast iron skillet over medium-high heat. Add the steak and sear it for 3-4 minutes per side, or until done to your preference, but don't overcook it. Remove the steak from the skillet and set it aside to rest for 5 minutes before slicing. Meanwhile, prepare your bread.

To Assemble

• Prep. Preheat the oven to 450ºF. Cut the baguette in half by length to open it.
• Top with cheese and onions. Place the baguettes, cut side up, onto the baking sheet. Top the bottom part of the baguette with the caramelized onions and Gruyère cheese. Bake until the cheese melts.
• Add steak. Slice the steak thinly and place the slices over the melted cheese, then add the spiced mayonnaise and mixed greens.
• Finish and serve. Close the sandwich with the top part of the baguette, then cut it in half or into fourths. Serve.