Heat the vegetable oil in a large saucepan or Dutch oven over medium-high heat. Add the pork pieces and brown them on all sides to seal in the flavour.
Add enough water just to cover the meat. Skim off any impurities that rise to the surface. Add the salt, oregano, half an onion, and 2 garlic cloves. Reduce the heat to low, cover, and simmer gently for around 2 hours, or until the meat is tender. Make sure the water does not evaporate completely—add a little more if needed.
While the pork is cooking, drain the nopales and rinse them thoroughly under cold water. Place them in a bowl with fresh water for 10 minutes to remove excess brine. Drain and cut into small pieces if necessary.
Make the Chile Colorado. Place the tomatoes, half an onion, 2 garlic cloves, and dried chillies in a small saucepan. Cover with water and cook for around 10 minutes until softened. Remove the ingredients from the water and, using a bit of the cooking water, blend until smooth. Pass the sauce through a sieve to remove any skins. Set aside.
Once the pork is cooked, add the prepared chile colorado sauce to the pot. Simmer together for around 15 minutes to allow the flavours to meld.
Stir in the prepared nopales and cook for an additional 5–10 minutes until heated through.
Serve hot, ideally with warm tortillas, rice, or beans.
