Taken from Napoleon’s Everyday Gourmet Grilling.
Season beef with Steak Spice, massaging well into the meat. Cover and set aside.
Preheat a sauté pan over medium-high heat on the side burner of the grill and melt the butter. Sauté the garlic and onions for 2 to 3 minutes and then add the mushrooms. Sauté for 10 to 15 minutes, stirring frequently, until the liquid has all evaporated and the mushrooms are tender and browned.
Preheat the grill to medium-high (450˚F/230˚C).
Grill steaks for 2 to 3 minutes a side for medium rare.
Top each steak with a quarter of the mushrooms mixture and 2 slices of Brie. Close the lid and grill until the cheese starts to melt, 1 to 2 minutes.
Lightly toast the baguette slices, cut side down. Top with arugula and then the cheese steak.