• Preheat a gas or charcoal grill to a medium-high temperature (about 400°F).

Prep the steaks for grilling:

• Whisk the balsamic vinegar and 1 tablespoon of the olive oil together, then drizzle half the mixture over one side of the steaks and spread evenly with the back of a spoon. Season with kosher salt and pepper, then flip the steaks and repeat the process. Set aside for 20 minutes to come to room temperature.

Sauté the onions and mushrooms:

• While the steak is marinating, heat the remaining 2 tablespoons of olive oil in a frying pan over medium heat. Add the onion and sauté until soft and translucent, 3 to 4 minutes.
• Add the mushrooms and continue cooking until the mushrooms are nicely browned and the onion is caramelized, 8 to 12 minutes longer. Season to taste with salt and pepper, then remove from the heat and set aside.

Grill the steaks:

• Oil the grill grates, add the steaks, and grill to desired doneness, 2-1/2 to 3 minutes per side (depending on thickness) for medium-rare. Remove the steaks to a cutting board and leave the grill on.
• Let the steaks rest for at least 5 minutes, then cut them into very thin (1/8-inch) slices.

Finish the sandwiches:

• Slice the baguette in half horizontally and place it on a sheet of heavy-duty aluminum foil (see notes). Line the bottom of the baguette with spinach leaves, add the steak slices and top with the onion-mushroom mixture.
• Add the blue cheese, top with the other half of the baguette, then fold up the sides of the foil, and place the sandwich on the grill until the cheese is softened and creamy, 2 to 3 minutes.
• Transfer the sandwich to a cutting board, compress gently, then cut it into 4 equal portions and serve.