1. Preheat oven to 250 degrees Fahrenheit (120 degrees Celsius).
2. Spread sliced onions and garlic cloves in a 9×13-inch (23 x 33-centimeter) baking pan.
3. To prepare the rub: Place all rub ingredients in a large bowl and blend with an immersion blender. Rub the entire surface of the meat with the paste and place into the baking pan.
4. Bake for six to eight hours. Remove from oven and slice. (It’s easier to slice it when it’s cool, but it’s not totally necessary in this case.)
5. To prepare the crumbs: Heat oil in a pan and sauté shallots until translucent. Add bread crumbs and stir to coat. Let heat in oil for a few minutes until slightly toasted, then add salt.
6. Cool and place crumbs in a bag. Shake with rosemary sprig (or
basil) and lemon zest. Store in a cool place for a few hours or
up to three days. Remove rosemary and discard. (If you want a
stronger rosemary taste, you can add some dried rosemary.)
7. Place sliced meat on a platter or on individual plates. Top with
crumbs and serve.