Preheat broiler to high with a rack in middle of oven.
Microwave potatoes, covered, until heated through, 3 to 4 minutes. Stir in cream an
Heat oil in a 10-in. cast-iron skillet over medium-high heat. Cook mushrooms, stirring occasionally, until softened and liquid has mostly evaporated, about 10 minutes. Add onion, carrots, rosemary, salt, and pepper; cook, stirring often, until onions and carrots are tender, about 10 minutes. Stir in garlic and cook 30 seconds. Add wine and cook until completely evaporated, about 3 minutes. Sprinkle vegetables with flour; cook, stirring constantly, 1 minute. Still stirring, gradually add broth. Cook, stirring often, until thickened, 5 to 6 minutes.
Remove skillet from heat. Spread mashed potatoes over vegetables to completely cover them. Broil until lightly browned, 3 to 4 minutes. Sprinkle with parsley.