Remove skin from pork and trim off any excess fat. Combine the mustard, cumin, coriander, brown sugar, ½ tbsp salt and 1 tsp pepper. Rub the mix onto the pork, covering all areas. Allow to sit for 30 minutes.
Preheat oven to 150°C.
Heat oil in a large Dutch oven over medium heat until almost smoking. Add pork and brown on all sides, 5 to 6 minutes total. Set aside.
Add onion, garlic and a pinch of salt. Fry for 3-4 minutes, stirring constantly, until beginning to brown, then add lager. Scrape bottom to release any stuck bits and simmer for a minute. Add stock and place pork back into pot. Bring to a simmer, cover with a lid, then transfer to the oven. Cook, turning the pork every hour, until it is tender, about 3½-4 hours.
For the coleslaw, combine the cabbage, carrot and celery seeds. Whisk together remaining ingredients, check seasoning, then gradually add the dressing to the vegetables.
For the mayo, combine ingredients, adding chipotle powder to taste.
Serve the pork at the table with mayonnaise, coleslaw and fresh milk buns, for everyone to make their own burgers.