With the rack in the middle position, preheat the oven to 275°F (135°C).
In a bowl, combine the garlic, herbs, fennel and red pepper flakes. Set aside.
On a work surface, unroll the roast. If needed, cut the thickest part of the roast to open like a book and create an approximate rectangle. Season the meat with salt on all sides. Rub the herb and spice mixture on the inside of the roast, roll it closed and tie with twine.
In a large French or Dutch oven or an ovenproof pot with a lid, add the oil and the roast. Place the onions around the meat. Season with pepper. Cover and bake for about 3 hours and 30 minutes or until the meat is fork tender. Remove the lid and finish cooking under the broiler to brown the roast.
Remove the onions and the roast and set aside on a plate. Over high heat, bring the cooking juices to a boil and reduce by half.
Slice the meat and drizzle with the cooking juices. Serve with Fennel and Kale Gratin with Parmesan Croutons.