Rinse guajillo chiles. Heat a large skillet over medium-high heat, add chiles, and cook, turning, until fragrant, about 40 seconds. Transfer chiles to a bowl, cover with hot water, and soak until soft, about 25 minutes. In the same skillet, toast avocado leaves, turning once, for 30 seconds. Crumble, then set aside. Drain chiles and remove and discard stems and seeds. (It is best to wear rubber gloves when handling chiles because oils can irritate skin.)
Place chiles and avocado leaves in a blender. Add 1 tsp. oregano, 1⁄2 tsp. thyme, cloves, and 1⁄2 cup water. Purée until smooth. Press through a sieve into a small bowl.
Rinse chicken and pat dry. Rub inside and out with chile purée. Place chicken, breast side up, in a roasting pan, cover, and set aside in refrigerator to marinate while you prepare stuffing.
Place potatoes in a pot of salted water and cook over medium-high heat until tender, about 10 minutes. Drain and set aside. Place raisins in a small bowl, add sherry, and soak until plump, about 15 minutes.
Heat lard in a large skillet over medium-low heat. Add onions and garlic, and cook, stirring, until onions are soft, about 15 minutes.
Add tomatoes to onions and cook until juices begin to evaporate, 3-5 minutes. Increase heat to medium-high, add beef and remaining oregano and thyme, and cook, breaking up beef with the back of a spoon, until brown, about 10 minutes. Stir in potatoes, raisins (with sherry), and olives. Cook until flavors blend, about 5 minutes, then season with salt and pepper. Set aside to cool.
Preheat oven to 350°. Spoon stuffing into chicken and tie legs together with kitchen string. Roast, uncovered, until juices run clear, about 1 1⁄2 hours, basting occasionally. Allow chicken to rest 10 minutes before carving. Serve warm with stuffing.