Whisk the hoisin, vinegar, garlic, salt and pepper in a large bowl. Add pork and marinate for at least 10 minutes or up to 24 hours.
Heat 2 tablespoons of oil in a large non-stick skillet over high heat. Remove the pork from the marinade using tongs (reserve marinade) and stir fry until browned, about 4 minutes. You may need to do this in smaller batches so you don't cool your pan down too much, just add more oil between batches as needed.
Tranfer the pork to a bowl. Add 3-4 tablespoons of water to the pan. Scrape up any bits that may be stuck to the bottom and pour the pan juices over the pork in the bowl.
Add the remaining 2 tablespoons of oil to the pan, when hot, add mushrooms and stir fry until slightly golden, about 2 minutes. Add the coleslaw mix and cook until wilted, about 3 minutes. Add the pork, the reserved marinade and the scallions (reserving some for garnish).
You can serve this one of three ways: Over cooked rice, in a bib lettuce cup or my favorite in a flour tortilla (trust me it's delicious). Enjoy!