Place all the sauce ingredients into a food processor or blender and whiz until smooth (ish)
Remove and discard the thick layer of fat from the pork.
Mix the spice rub ingredients together (except the meat!) in a bowl, and rub all over the pork.
Place the pork in a dish and pour over the sauce. Leave to marinade for at least 3 hours (or overnight if you can.)
Place the sliced onions into the bottom of the slow cooker. Add the marinated pork and all of the sauce. Cook on low for at least 12 hours (we cooked ours for about 18 hours!)