Prep the steak. Using a boning knife, slice the meat horizontally on the long side of the steak, but do not cut all the way through, leave about ½ inch uncut. You want the steak to open like a book. Make sure the top half is the same thickness as the bottom half. Open up the steak so that it's flat. Use a meat mallet to pound down the steak to a uniform thickness about ½ inch in thickness or thinner.
Make the marinade and marinate the steak. Add all the marinade ingredients to a large bowl and whisk to combine. Add the butterflied steak to the bowl and toss it around in the marinade to coat it all around. Cover the bowl with plastic wrap and refrigerated for 2 hours.
Prep the oven. Preheat the oven to 350°F.
Make the walnut pesto. In a food processor add all the pesto ingredients and pulse a few times, don’t over process as you don’t want a smooth consistency. Taste for salt and pepper and add more if needed.
Assemble the steak. Remove the steak from the marinade; reserve marinade. Place the steak flat on a cutting board. Spread the pesto evenly over the steak, leaving about ½ inch free on all sides of the meat. Layer the steak with prosciutto slices, followed by roasted peppers, then provolone cheese slices. Sprinkle with salt and pepper. Roll up the steak jellyroll style. The grain of the steak should run on the length of the roll. Tie the roll with kitchen twine or string, every 2 inches.
Sear the steak. Add the olive oil to an oven safe skillet and heat over medium-high heat. Add the steak roll and sear quickly on all sides until brown.
Roast the roll. Transfer the skillet to the preheated oven and roast for 30 to 40 minutes or until the interior of the meat registers at 130°F when tested with an instant read meat thermometer.
Rest the meat and slice. Remove the skillet from the oven and let the steak rest for 10 minutes. Remove the string from the roll and cut into slices.