How to Make It
Preheat the oven to 325°. Put the lamb in a deep roasting pan and add 4 cups of the water, the bell peppers, diced onion and the tomatoes and their juices. Season with salt and pepper. Tie the cloves, coriander seeds and 1 cinnamon stick in cheesecloth and add it to the pan. Cover tightly with foil and braise the lamb for 2 1/2 to 3 hours, until an instant-read thermometer inserted in the thickest part registers 180°. Discard the spice bundle.
Meanwhile, in a medium bowl, combine the sliced onion with the vinegar, sugar, 2 tablespoons of kosher salt and the remaining cinnamon stick. Let the onion stand at room temperature for at least 2 hours and up to 4 hours.
Transfer the meat to a cutting board and let cool slightly. Transfer the peppers, onion and tomatoes to a bowl and reserve. Strain the cooking liquid into a heatproof measuring cup and skim off the fat. Transfer the cooking liquid to a large, deep skillet and simmer over moderate heat until reduced to 1 1/2 cups, about 20 minutes. Return the peppers, onion and tomatoes to the sauce.
When the lamb is cool enough to handle, remove the strings from the roast and shred the lamb, discarding any excess fat. Add the lamb to the sauce and season with salt and pepper.
In a large bowl, combine the masa harina with the remaining 2 1/2 cups of water and stir with a wooden spoon until a stiff dough forms. Add a few tablespoons of water if the dough seems very dry and crumbly. Add the olive oil and knead the dough until smooth. Divide the dough into 20 pieces and roll each one into a ball. Keep the dough balls covered with a lightly moistened towel; working with a few at a time, flatten the balls into 3-inch tortillas, a scant 1/3 inch thick.
Heat a cast-iron griddle or skillet until hot. Add the tortillas in batches and cook over moderately high heat, turning once, until lightly browned and slightly crisp on the outside, 5 to 6 minutes. Serve the shredded braised lamb over the warm tortillas, garnished with the pickled onions and cilantro leaves.