Preheat oven to 275°F.
Cut the pork butt roughly into 3 inch pieces.
Place the cut up pork in an oven-safe casserole dish (or dutch oven), along with the onions- mine was a 9 inch square dish that fit all the pork snugly.
Sprinkle the oregano, cumin, coriander, sugar and salt over the meat and mix it in well until all of the pork is coated well with the spices.
Tuck the garlic cloves and jalapeno inside the pork pieces.
Squeeze the lime juice from the quartered lime wedges on top, and tuck the wedges inside the pork pieces as well.
Mix the extra lime juice, tequila, milk and oil in a small container and pour it over the pork pieces. Press the pork into the marinade to make sure it's all snugly packed.
Cover the dish tightly with foil, and slow roast in the oven for about 3.5 hours. Remove from the oven and lift the edge of the foil and check if the meat is fork tender (use a fork and pull apart a piece of meat). If not, cover and return to the oven for another 30 minutes (the exact time depends on the oven and the dish you use - my oven usually takes 3.5 hours, but I have left it in there for up to 4 hours occasionally).
Remove from the oven. Let it rest for about 30 minutes.
Remove the foil and the pieces of pork from the dish. Strain the liquid ("stock") left in the dish and discard the onion, garlic, jalapeno and lime peel.
Gently skim off oil from the surface of the stock and return that oil to the pork (you can use a spoon or an oil separating jug if you have one, to do this). Add another ¼ cup of the remaining stock to the pork as well.
Shred the pork using two forks. Then let the meat rest a little while and soak up the liquid (overnight would be best, but not necessary).
Before serving, prepare the broiling rack (or an oven tray) by covering it with some foil.
Turn on the broil setting in your oven.
Place the pork on the tray, spread it out and let it crisp up in the oven (mine took no longer than 10 minutes, so keep an eye on it!). Mix up the carnitas carefully and broil it again one more time (to get more browned, crispy bits). Now they are ready to be served.
These slow roasted pork carnitas can be served as is, with some rice and beans. Or in tacos, enchiladas, quesadillas, burritos, etc.