To make a simple mashed turnip casserole, boil 2 pounds of peeled, chopped turnips in enough water to cover them. Simmer the vegetables for 20 to 25 minutes, drain the pot, and mash the turnips to a smooth consistency. Sauté 1 small chopped onion in 1 tablespoon of butter in a skillet set over medium heat for three to five minutes, or until the onions are translucent and tender.
Stir the cooked onions along with 1/2 cup milk, 1 cup grated cheddar cheese, 3 tablespoons of butter into the mashed turnips. Season the mixture with salt and pepper to taste before transferring the mixture to a greased baking dish. Sprinkle 1/3 cup breadcrumbs and 1/2 cup grated cheddar cheese over the top, and bake the casserole for 30 minutes at 350 degrees Fahrenheit.
Alternatively, make a Finnish-inspired "lanttulaatikko," a mashed turnip casserole that balances the sharp flavor of the turnips with a hint of sweetness and spice. Boil three large peeled, chopped turnips for approximately 20 minutes, or until the vegetables are tender. Drain the pot, and mash the turnips. Beat 2 eggs into the mashed turnips, along with 1 tablespoon of sugar, 2 tablespoons or butter, 1/4 cup heavy cream, and 1/2 teaspoon of ground ginger. Season the mixture with salt and pepper, transfer it to a greased casserole dish, and bake it for approximately one hour at 350 degrees Fahrenheit.