Preheat oven to 375°F (190°C). Grease a 9 x 13 inch (3.5 L) baking dish with butter.
Scrub potatoes (no need to peel) and cut into chunks; place in a large pot. Peel and discard thick peel from rutabaga. Cut rutabaga into chunks (about the same size as the potato chunks); add to pot. Add stock, salt and nutmeg. Add as much water as needed to bring liquid in the pot just below the level of the vegetables; bring to a boil. Boil gently for 10 to 15 minutes or until vegetables are tender. Drain, reserving 2 cups (500 mL) of the cooking liquid.
Transfer vegetables back to the pot. Mash until smooth, adding enough of the reserved cooking liquid to help vegetables become a chunky purée. Transfer to a prepared dish; smooth into an even layer and sprinkle with cheese. Bake for 15 to 20 minutes or until cheese is bubbly and browned.
(Make-ahead: Cook and purée vegetables; transfer to baking dish and let cool completely. Cover well and keep refrigerated for up to 2 days. Sprinkle with cheese and bake, adding 10 minutes to baking time.)
Nutrients per serving: 187 calories, 9 g protein, 8 g fat (5 g saturated fat), 19 g carbohydrates (2 g fibre), 27 mg cholesterol, 289 mg sodium
Best Health tip: Gruyère is delicious, but you can substitute it with any strong cheese, such as Cheddar and Swiss.