Place potatoes in a large saucepan; add water to cover. Bring to a boil. Reduce heat to medium; cook, uncovered, until tender, 10-15 minutes., Meanwhile, in a large skillet, cook and crumble beef with mushrooms and onion over medium-high heat until no longer pink, 6-8 minutes; drain. Stir in flour, 1/2 teaspoon pepper and 1/4 teaspoon salt until blended. Gradually stir in broth; bring to a boil. Reduce heat; simmer, uncovered, until thickened, about 5 minutes, stirring occasionally., Drain potatoes; return to pan. Mash potatoes to desired consistency, adding butter, cream and the remaining salt and pepper. Spoon into bowls; top with beef mixture. Sprinkle with fried onions.