When it comes to radiating warmth at the dinner table, nothing beats a meal-in-one casserole. What's more, this dish can be assembled a day ahead, goes straight from the oven to the table, and offers help-yourself allure in abundance.
Preheat oven to 425 degrees. Cover potatoes with 2 inches water in a pot; bring to a simmer. Cook until tender, about 17 minutes. Drain and return to pot. Mash with 2 tablespoons oil; season with salt.
Place beef in a 10-inch cast-iron skillet over medium-high and cook, stirring occasionally and breaking up large pieces with a spoon, until browned, about 11 minutes. Transfer to a plate using a slotted spoon; pour off excess fat in skillet.
Add 1 tablespoon oil, mushrooms, and onion to skillet; cook, stirring occasionally, until browned, about 6 minutes. Add beer; cook until reduced, about 3 minutes. Whisk together cornstarch, mustard, and broth in a bowl; add to skillet and bring to a boil. Return beef to skillet; season with salt and pepper. Top with mashed potatoes. Bake until bubbling, 25 to 30 minutes. Serve hot.