Step 1

Trim any excess fat from the steak, season with salt and pepper and drizzle each side with 1/2 tablespoon balsamic vinegar. Set aside for 20 minutes to come to room temperature.

Step 2

While the steak is marinating, heat the olive oil over medium heat. Add the onion and sauté, stirring frequently, 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.

Step 3

Preheat a gas or charcoal grill to a medium-high temperature (about 400°F). Oil the grates, add the steak and grill to desired doneness, 2-1/2 to 3 minutes per side for medium-rare (depending on thickness). Leave the grill on. Allow the steak to rest for 5 minutes, then cut into very thin (1/8-inch) slices.

Step 4

To assemble the sandwiches, place the baguette on a sheet of heavy-duty aluminum foil. Line the bottom of the baguette with spinach leaves, add the steak slices and top with the onion-mushroom mixture. Add the blue cheese and place the sandwich on the grill until the cheese melts, 2 to 3 minutes. Top with the other half of the baguette, compress gently and cut into 4 equal portions.