To make the lechon preheat the oven to 350F. Spread the celery, onion, carrots, garlic and thyme in the bottom of a roasting pan; place the lechon on top of the vegetables.
Add the orange juice, then squeeze the oranges over the lechon. Cover the pan with foil and roast for 4 to 5 hours, or until thoroughly cooked.
Remove the foil and let the lechon cool down and rest. Remove the skin and set aside for future use; remove all the meat from the bones and shred it. Reserve the bones for the salsa.
To make the orange salsa bring all the ingredients to a boil in a medium saucepan. Continue to cook for 20 minutes. Remove from the heat and strain through a fine-mesh sieve; check seasoning.
To make the guacamole scoop the pulp out of the ripe avocados into a bowl, set aside. In another bowl combine the diced onion and jalapeño with the lime juice and a pinch of salt, let marinate for 3 minutes. Then add the diced tomato, cilantro and avocados. Mash the ingredients together and check seasoning.
To make the nopales salad prepare and heat a grill to medium heat. Remove the "thorns" of the cactus leaves: hold each leaf at the base, then run a very sharp knife over the surface of the leaf, on each flat side, to cut off the thorns. Try to leave behind as much green skin as possible. Cut around the edge of each cactus leaf to remove any thorns that stick out.
Rub each cactus leaf with 1/2 teaspoon of olive oil and 1 teaspoon of salt. Grill the cactus until they are seared and crisp. They should be pale-green, with blackish spots, about 3 minutes on each side. Allow cactus leaves to cool slightly, about 3 minutes, and then cut them into 1/2-inch squares and place in a bowl.
Add the chopped red onions, vinegar, epazote and 1/2 teaspoon salt to the bowl. Mix all together. Let salad stand at room temperature for 1 hour. Add the remaining 3 tablespoons of olive oil to the salad and mix well. Sprinkle with the crumbled queso fresco.
To serve, make a circle of guacamole on each plate. Stack a generous portion of the shredded lechon on top and drizzle with the salsa. Then pile on the nopales salad, and top with a pork rind. Serve with the warmed corn tortillas and the remaining salsa on the side.