Put a large, heavy bottomed pot on medium high heat. Preheat your oven to 140°C.
Brown the beef shin pieces on both sides in batches. Trying to do them all at once will release too much liquid and they won't sear. Set aside when browned.
When all the beef shin is browned and removed, add in the whole anchos. Cook for 30 seconds then add the onion and season with salt and pepper. Cook for a further 3-5 minutes.
When the onion has started to soften and has lifted the browning from the beef off the pan, add the peppers, jalapenos and garlic.
Add in the spices and mix thoroughly. Cook for 2-3 minutes until they become fragrant then add the tomato puree. Cook for a further 2-3 minutes until you start to see some oil separate from the puree.
Add in the crushed tomatoes and stir well so the vegetables and spices incorporate evenly. Pour in the stock and stir again to mix.
Add the beef shin pieces back into the pot, ensuring they are all submerged. If there is not enough liquid in the pan, add a bit of water.
Put the lid on the pot and put it into the oven for 4 hours. Check on it every hour to ensure that the liquid level is still staying above the meat. Add a little more water if needed.
After 4 hours, the meat should fall apart with a pair of tongs very easily. Shred the beef in the pot and remove the ancho chillis. By this point, most of the chillis will have broken up an mixed in, so it should just be the stems you need to remove.
Increase the temperature up to 180°C. Add the pot back in and cook for a further 30 minutes. Remove from the oven and give a final check for the taste and consistency. The meat should be delicious and tender, but if there is not enough sauce for your liking then mix in a little bit of water to loosen it.
