To butterfly the tenderloins: make a deep lengthwise cut down the center, being careful not to cut all the way through. Open the tenderloin up like a book. Place the opened tenderloin between 2 pieces of plastic wrap. Pound with a meat mallet until 1/4-inch thick. Repeat with the other tenderloin.
To make the stuffing: heat the olive oil in a large skillet over medium heat. Add the onion and cook until lightly golden, about 6 to 8 minutes. Add the garlic and cook 1 minute more. Add the kale, vinegar, honey, and water. Increase the heat to high and continue to cook, stirring constantly, until the kale is wilted, about 2 minutes. Drain and set aside.
Lay 2 slices of ham or prosciutto over each butterflied tenderloin. Top with half of the kale stuffing, leaving a 1/4-inch border. Carefully open the peppers and lay a single layer over the kale. Roll up each tenderloin and tie at 2-inch intervals with butcher's twine.
Preheat the oven to 450 degrees F.
Sprinkle the tenderloins all over with salt and pepper. Heat a heavy ovenproof skillet over medium-high heat and add the olive oil. Place the tenderloins in the pan and brown on all sides for about 3 to 5 minutes. Transfer the pan to the oven and roast. After 15 minutes, begin checking the internal temperature with an instant-read thermometer, checking every 5 minutes, until the pork registers 140 degrees F to 145 degrees F. When done, remove the tenderloins from skillet to a platter, cover loosely with foil, and set aside to rest for 10 minutes. Remove the twine from the tenderloins, cut them into 1/2-inch-thick slices, and serve with Romesco sauce.
For the Romesco sauce: In a food processor fitted with the metal blade, grind the almonds until they form a thick paste. Add the bread crumbs, olive oil, peppers, and capers and process until smooth. Taste for salt and pepper. Transfer to a serving bowl.
Recipe courtesy of Bruce Aidells, Complete Book of Pork, HarperCollins, 2004