In a medium frying pan over medium-low heat, cook pancetta until lightly browned and crisp, about 15 minutes. With a slotted spoon, transfer to a bowl. Add shallots to fat in pan and cook over medium heat until softened, about 4 minutes. Pour shallots and fat into pancetta and mix. Let cool to room temperature.
Butterfly-cut breasts so that they're evenly thick: Unfold each whole breast. Carefully remove skin and set it aside. Working with one whole breast at a time, arrange thickest part farthest from you. With a sharp knife, slice horizontally almost all the way through thickest part of each side, then unfold the flaps of meat as though opening a book. Repeat with second breast.
Working with one breast at a time, lay it between two sheets of parchment and pound with flat side of a meat mallet until about 1/2 in. thick. Season generously all over with salt and pepper.
Mince garlic, then sprinkle with coarse salt. Mash to a paste with flat side of a chef's knife (or use a mortar and pestle). Scrape into a bowl.
Spread skin from 1 breast out on a work surface, outer side down. Arrange 1 flattened breast on skin. Rub half of garlic paste over meat. Repeat with skin and meat of second breast and remaining garlic paste.
Stir parsley, 3 tbsp. minced rosemary, the piment d'Espelette, and fennel pollen into pancetta and shallots. Season lightly with salt and spoon evenly onto breasts. Working with one breast at a time, and starting with end closest to you, snugly roll the whole thing into a roast, skin side out. Using butcher's twine, tie at 1-in. intervals.
Preheat oven to 400°. Pat roasts dry, then season all sides with coarse salt. Heat 1 large or 2 medium cast-iron skillets over medium-high heat. Swirl in enough oil to coat bottoms of skillets. Add roasts and brown well on all sides, about 15 minutes.
Arrange carrots and celery down center of a shallow roasting pan big enough to hold both roasts. Set roasts on top of vegetables, then rub all over with softened butter. Pour wine into pan.
Roast uncovered, basting every 20 minutes or so with wine and juices, until an instant-read thermometer inserted in thickest part of roasts registers 155°, 45 minutes to 1 1/2 hours. Transfer roasts and vegetables to a carving board and let rest 10 to 15 minutes (temperature will rise to about 160°).
Meanwhile, make sauce: Set roasting pan over 2 burners on stove. Add a large sprig of rosemary and one of thyme to fat in pan, pour in stock, and bring to a simmer over medium-high heat, scraping up browned bits. Meanwhile, mix cornstarch with 3 tbsp. water. Whisk slurry into roasting pan, then add remaining 1 tsp. minced rosemary and 1 tsp. minced thyme. Simmer sauce, whisking, until thickened, 5 minutes. Strain into a bowl or gravy boat.
Snip twine on roasts with kitchen shears and remove. Slice roasts crosswise into 1-in. slices, then lay slices back in roasting pan with carrots and celery. Serve with pan sauce.
Serves 10 to 12, with leftovers