Set up your grill (charcoal or gas) for direct grilling and heat to medium-high (400° F), leaving 1/3 fire-free as a safety zone.
Put half the steak in a single layer in the center of a 16-inch-long piece of Reynolds Wrap® Non-Stick Aluminum Foil. Top with half the peppers, mushrooms and onions. Season with salt and pepper. Place 1 tablespoon of butter on top of the meat and vegetables. Repeat for the remaining package.
Bring the long edges of the foil together and fold twice. Fold the short edges to completely enclose the meat and vegetables. Repeat for the remaining package. Transfer each foil packet to a rimmed baking sheet. Lightly butter the cut sides of the baguette or rolls with the remaining butter. Set aside.
Arrange the foil packets, seam-side up, on the grill grate over direct heat. Grill for 6 minutes. Protect your hands with grill gloves, then using tongs, turn the packets over and grill for 2 minutes more. Move the packets to the safety zone, seam side up. Carefully open the packets to reveal the meat and vegetables. (The emerging steam will be very hot.)
Top each with half the provolone, overlapping the slices. (There is no need to reseal the packets.) Cover the grill with the lid for 1 to 2 minutes to melt the cheese. Remove the lid, then grill the bread, cut sides down, 30 seconds to 1 minute. (The bread will toast quickly, so watch carefully.) Transfer the foil packets and the grilled bread to a rimmed baking sheet.
Using a wide spatula, carefully transfer the meat, vegetables, and cheese from each foil packet to the bottom side of a baguette. Serve immediately.