How to Make It
Preheat the oven to 350°. In a blender, combine the rosemary and lavender with 6 of the garlic cloves and 1/2 cup of the olive oil; puree until smooth.
Season the tenderloin with salt and pepper. Make 4 evenly spaced 1-inch-deep slits in the tenderloin and insert the remaining 4 garlic cloves. Rub 2 tablespoons of the herb oil over the tenderloin.
Place the pork belly skin side down on a work surface, season with salt and pepper and rub with the remaining herb oil. Place the pork tenderloin on top of the pork belly and roll the belly tightly around the tenderloin. Tie with kitchen twine at 1-inch intervals. Rub the remaining 2 tablespoons of olive oil all over the outside of the pork belly.
Combine the carrots, onions, fennel pieces and Marsala in a large roasting pan. Top with the pork and roast for 2 1/2 to 3 hours, basting occasionally, until the skin is deeply golden and crispy and an instant-read thermometer inserted in the thickest part of the pork registers 150°. Transfer the pork to a cutting board and let rest for 20 minutes. Stir the fennel fronds into the vegetables. Using a serrated knife, slice the porchetta crosswise about 1 inch thick and serve with the vegetables.