Pat meat dry. Season on all sides with salt and pepper. In a heavy casserole brown pork on all sides in oil and butter over medium-high heat. Remove pork to a plate. Reduce heat to low, stir in onions, and cook, stirring occasionally, until softened (about 5 minutes). Return pork to pan and add any juices from plate. Add cinnamon, stock, and the water, pushing cinnamon stick into liquid. Bring to a boil. Reduce heat to low, cover, and simmer, turning pork over from time to time, 1-1/2 hours. Add prunes to liquid, cover, and cook until meat is very tender when pierced with a knife (meat should register an internal temperature of 150°F. on a meat thermometer) and prunes are tender (about 30 minutes). Stir agave into pork, cooking liquid and cook, uncovered, over low heat, basting pork often, for 5 minutes.

Transfer meat to a plate with 2 wooden spoons, leaving as much of the chopped onions as possible in casserole. Remove prunes to a bowl with a slotted spoon. Skim as much fat as possible from cooking liquid. Boil liquid over high heat, stirring often, until it thickens (about 5 minutes). Transfer pork to a carving board and discard strings. Cut pork in slices about 1/2 inch thick. When sauce has thickened, add lemon juice. Add more salt and pepper, if needed. Discard cinnamon stick. Return prunes to casserole and reheat over low heat. Set pork slices gently on top, cover, and reheat over low heat 5 minutes.

Pork can be kept, covered, up to 2 days in refrigerator; reheat pork and prunes in sauce over low heat, covered. Arrange pork slices on a platter or on plates, and spoon sauce and prunes over and around slices.