Trim off all but a thin layer of fat from the pork.
The slow-fry method: Melt the lard in a large (4-quart), heavy saucepan over medium-low heat. When it has melted but is not yet very hot, add the pork, water and lime zest. Cook with the lard at a gentle simmer, turning the pork occasionally, for about 40 minutes, until it is barely tender. Raise the heat to medium-high. If all of the water hasn't yet evaporated, the lard will come to a rolling boil, which will eventually diminish into small bubbles as the water evaporates. After the change occurs, the carnitas will need about 10 minutes to brown. Watch them carefully and remove when they are a light golden brown. Drain on paper towels and sprinkle with salt.
The boil-then-fry method: Place the meat in a single layer in a wide, heavy saucepan, add enough water to cover the meat by 1/2-inch, measure in the salt, and set over medium heat. Simmer, partially covered, turning the pork occasionally, until the meat is barely tender, about 40 minutes. Uncover, raise the heat to medium-high, and quickly boil away the liquid.
When you hear the meat begin to fry in its own rendered lard (once the water is gone), turn the heat down to between medium and medium-low. Let the pork fry, turning frequently, until evenly browned, about 30 minutes. Remove the ribs from the pan, drain on paper towels, and sprinkle with a little salt.
While the meat is cooking, prepare the guacamole, scoop it into a serving bowl and garnish. Serve the crispy ribs on a warm serving platter and pass the guacamole to eat along with them. Or cut the meat off the bones and roll with the guacamole into tacos.
In a medium-size bowl, mix the finely chopped onion and chiles with the optional tomato, garlic, and cilantro.
Close to the time you are going to serve, halve the avocados lengthwise by cutting from the stem to flower ends, around the pits. Twist the avocado halves in opposite directions to loosen the meat from pits, then scoop out the pits, and reserve. Scrape the avocado pulp from the skins and add it to the bowl.
Using your hand or a spoon, roughly mash the avocado while mixing in the other ingredients, making a course, thick mass. Flavor with salt, then enough lime juice to add a little zing, if you wish. Return the pits to the guacamole and cover with a sheet of plastic wrap pressed directly onto the surface of the mixture. Set aside for a few minutes to let the flavors blend.
The guacamole is very attractive in a pottery bowl or Mexican mortar, sprinkle with chopped onion, cilantro, radish slices and/or queso fresco (cheese); radish roses really dress it up.
Yield: about 3 cups, serving 6 as an appetizer, 12 to 15 as a dip