In a large saucepan, cover potatoes with salted water. Boil for 20 min. or until pieces yield when pierced with a fork. Drain very well.
Warm butter and milk in a small saucepan on low heat or in the microwave. Mash the potatoes to your preferred consistency; stir in the warm milk, salt, pepper and chives. Keep potatoes warm until the pork is ready to serve.
Meanwhile, mix the flour with half of the salt and the pepper and dredge pork slices. Heat half the oil in a large skillet on medium-high. Sauté the pork 3 to 5 min. per side. Remove to a plate. Heat the remaining oil, add mushrooms and cook 5 min.
Pour in the broth and bring to a boil while scraping with a wooden spoon. Reduce the liquid slightly. Return the pork to the skillet; cook 5 min. Turn off the heat and mix in the sour cream and rosemary. Season with the remaining salt and pepper, if needed, and serve with the mashed potatoes.
Slicing pork tenderloin into discs cuts down on cooking time.