Cover potatoes with water in 3-quart saucepot. Bring to a boil over hight heat. Reduce heat to medium and cook until potatoes are very tender, about 10 minutes; drain.
Meanwhile, melt 2 tablespoons Country Crock® Spread in 12-inch nonstick skillet over medium-high heat and cook mushrooms, onion and rosemary, stirring frequently, until dark golden brown, about 10 minutes. Remove from heat and stir in milk.
Drain potatoes and return to saucepot. Add remaining 2 tablespoons Spread; mash. Stir in mushroom mixture and season with pepper.