Place the pork shoulder in a large resealable plastic bag. In a bowl, stir together the crushed red pepper flakes, black pepper, coriander, sugar, and 1 tablespoon of salt. Pour the spice mixture into the plastic bag, seal it, and shake it to coat the pork. Place bag in a shallow bowl and refrigerate overnight. 

Preheat oven to 325℉.

 Set the pork shoulder in a small roasting pan, and roast for 2 hours 15 minutes, or until an instant-read thermometer inserted into the thickest area of the meat reads 185℉. Remove from oven and let sit for 20 minutes.

Heat the olive oil in a large skillet over medium-high heat. Add the mushrooms, onions, and a pinch of salt. Cover and cook, stirring once or twice, until the onion is tender, about 5 minutes. Uncover and continue to cook, stirring occasionally, about 5 more minutes, until the mushrooms are tender and golden-brown. Stir in the vinegar and cook until almost evaporated, then stir in the butter and chives. Cover again, and reduce heat to very low to keep warm.

Slice the pork, and serve with the mushrooms on top.