To prepare the mole sauce: In a blender or food processor, blend tomatoes and garlic cloves, adding water if necessary. Strain and set aside. In a spice grinder pulverize the sesame seeds, peppercorns, oregano and cinnamon; set aside.
Make a lengthwise cut on 1 side of each ancho and guajillo chile and remove seeds. Put chiles in a bowl and cover with boiling water, letting soak for 20 minutes or until soft. Place chiles and soaking water in a blender. Puree, strain and set aside. In a medium stockpot, heat 3 tablespoons of canola oil. Add the dried spices stirring continuously. Add the two purees and bring to a boil. Add the chocolate, sugar, and salt to taste; let simmer for about 20 minutes.
To prepare the lamb shanks: Season the lamb shanks with salt and in a hot skillet sear shanks in oil on all sides. In a bowl, dip each shank in mole sauce, sprinkle with some of the vinegar, chopped onion and salt, then wrap in a banana leaf and then in aluminum foil. Place shanks in a roasting pan and bake in a preheated 325 degree oven for 3 1/2 to 4 hours, or until the meat is falling off the bone. Place shank on a plate and top with additional warm mole sauce.