Prepare the garlic bread: Place the butter, garlic, and cilantro in a mixing bowl and whisk to mix. Lightly butter each bread slice with the garlic butter…
Prepare the garlic bread: Place the butter, garlic, and cilantro in a mixing bowl and whisk to mix. Lightly butter each bread slice with the garlic butter and set aside.
Make the chipotle sauce: In a small bowl, combine the mayonnaise, chipotle, and adobo sauce and stir to blend.
Set up the grill for direct grilling and preheat to high. Ideally, you’ll be working on a wood-burning grill. Brush and oil the grill grate.
Grill the poblanos until charred on all sides, 4 to 6 minutes per side. Transfer to a cutting board and let cool. Scrape off any burnt skin, remove the stems, and cut the chiles in half. Scrape out and discard the seeds.
Skewer the onion slices crosswise on skewers. Brush on both sides with oil and season with salt and pepper. Grill the onions until browned on both sides, 2 to 3 minutes per side. Transfer to the cutting board, let cool, and remove the skewers.
Make a lengthwise cut in the tenderloin (do not cut all the way through) and open it like a book. Season the meat, inside and out, with salt and pepper. Spread the inside with half of the chipotle sauce. Lay the strips of poblano end-to-end in the pocket you cut in the meat. Top with grilled onions and slices of provolone. Close the meat around the cheese and vegetables and tie at 2-inch intervals with butcher’s string. Brush the outside of the tenderloin with olive oil and season generously with salt and pepper.
Grill the tenderloin until well-browned on the outside and cooked to taste, 4 to 5 minutes per side, 15 to 20 minutes in all for medium-rare (about 135 degrees). Transfer the beef to a cutting board and let rest for a couple of minutes. Remove the string, then cut the tenderloin crosswise into 1/2 inch thick slices.
Meanwhile, grill the garlic bread until browned on both sides, about 1 to 2 minutes per side.
To serve, spread the garlic bread with the remaining chipotle sauce. Top with grilled beef and serve as an open-faced sandwich.