The day before cooking, season the pork shoulder all over with salt.
Preheat the oven to 160C. Slice the head of garlic in half crosswise. Set a large pot over medium high heat and add olive oil. When it begins to shimmer, add the pork shoulder and brown on all sides (you may need to cut the shoulder into pieces if the whole shoulder doesn't fit into the pot - in that case, do this stage in batches until all pieces are browned).
When the meat is browned on all sides, remove and set on a plate. Save about 1 tbsp of pork fat (discard the rest) and add the onions and garlic, stirring for about 15 minutes.
Add the tomatoes, cumin seeds, bay leaves, dried chillies, smoked paprika and chipotle paste. Stir until combined. Sit the pork on top of the mixture and pour beer into the pot until it comes about halfway up the pork. The peppers and onions should be entirely covered (they will burn otherwise).
Bring to a boil, then put the pot (uncovered) into the oven for 6 hours. Check the meat every hour and make sure the liquid comes at least halfway up the meat; if not, add more beer. When the pork is tender and falling apart, remove the meat to a bowl and shred with two forks.
Remove the bay leaves from the aromatic mixture. Put the mixture into a food processor or blender and blend, then strain through a sieve. Discard any solids. This is a delicious sauce to have with your pulled pork - have this with nachos, tortillas, or spoon it over your tacos. Or, mix it back in with the meat.