This crowd-pleasing side dish is perfect for gatherings.
Saute pancetta in a skillet over medium heat until browned and crisp, about 8 minutes. Drain on paper towel.
Place potatoes in a large pot and cover with cold salted water. Bring to a boil and cook until fork-tender, about 12 minutes. Drain. Transfer back to pot.
Meanwhile, place heavy cream, milk, garlic, and rosemary in a medium saucepan. Bring just to a boil over high heat. Reduce heat to low and simmer until liquid reduces by half (about 20 minutes). Remove rosemary (its okay if a few stray leaves stay in the liquid) and pour mixture into pot with potatoes. Season with salt and coarsely mash.
Sprinkle with pancetta and serve immediately.
If making the night before, reheat potatoes over medium heat, adding more milk to get desired consistency. Then add pancetta.