Place the meat in a large soup pot or casserole and bring to a rolling boil over high heat. Skim off any foam. Add the halved onion, garlic cloves, 2 teaspoons salt, black peppercorns, bay leaves, oregano, coriander and cumin seeds, and stir well. Reduce heat to medium low, cover and cook for 1 1/2 hours, or until the meat is fork tender and shreds easily. Remove from the heat.
Remove the meat from the broth, set aside, and strain the broth. Reserve 1 1/2 cups of the broth for later use and store or freeze the rest if you want to use it for something else. Once the meat has cooled enough to handle, shred or chop into smaller pieces.
To make the seasoned cooked meat:
Heat the oil in a large wide casserole or skillet over medium heat. Add the onion and the potatoes, season with 1/2 teaspoon salt, and cook for about 10 minutes, stirring occasionally, until the onions have softened and the potatoes have begun to brown. Stir in the tomatoes and cook for 3 to 5 minutes, or until the tomatoes are soft and mushy. Add the chiles and the other 1/2 teaspoon salt and cook for another 3 to 4 minutes, stirring often, until the mixture is nicely seasoned and blended together. Stir in the shredded meat and reserved meat broth and cook until the broth is mostly absorbed, the potatoes have completely softened, and the mixture is nicely amalgamated and delicious. It should be moist but not wet.
To make the chimichangas:
Heat a comal, griddle or skillet over medium low heat for at least 3 to 4 minutes. Heat the flour tortillas a couple at a time (or 1 at a time if that’s all you can fit in the skillet or on the griddle in a single layer) for about a minute per side, until completely heated through. Remove tortillas and one by one, top with a couple of generous spoonfuls of the shredded cooked beef. Roll to enclose the filling, and after the first roll, fold in the sides and continue to roll, making an elongated burrito-shaped package.
Once the packets are ready, heat 2 tablespoons oil in a skillet over medium heat. When the oil is hot, place the chimichangas in the pan seam side down and fry for a minute on each side, until lightly colored. Serve garnished with lettuce, queso, and if desired crema, tomato, avocado and salsa.