Whisk 2 1/2 tbsp hoisin sauce, rice vinegar, minced garlic, ginger and salt and pepper. Pour over pork in a resealable plastic bag. Marinate in refrigerator for about 1 hr.
Remove the pork from the refrigerator and add to a bowl, reserving the marinade for later. Toss the pork with the arrowroot flour.
Heat 1 tbsp olive oil in a large skillet or wok over high heat. Add the pork and stir fry it for 3-4 minutes until browned. Set the pork aside on a plate. Add marinade to the skillet, scraping any browned bits from the bottom of the skillet and set aside.
Add egg whites to the skillet or wok and fry them quickly, then set aside for chopping. Add remaining oil, then stir fry the sliced mushrooms until golden, about 2-3 minutes. Add coleslaw and quickly cook until the coleslaw is slightly softened, about 1-2 minutes.
Return the pork and marinade to the skillet or wok and add 1/2 of the sliced scallions. Stir fry another 2 minutes and remove from heat.
Serve pork and slaw mixture on top of butter lettuce leaves. Top with remaining scallions and hoisin.