Prep the vegetables. Peel the rutabaga (and carrot, if using) and cut into uniform cubes about 1 inch across. Do the same with the potato. Even pieces cook faster and mash more smoothly.
Cook in salted water. Place all the cubes in a large pot and cover with cold water. Add a pinch of salt, then bring to a boil over high heat. Once boiling, reduce heat, cover, and simmer until the vegetables are very tender (about 20–30 minutes). You should be able to easily pierce them with a fork.
Drain and dry. Carefully drain the cooked vegetables in a colander. Let them sit for a minute so any excess water evaporates – this helps the mash stay thick and not watery.
Mash with butter. Return the drained veggies to the pot (off the heat). Add the butter and mash everything together using a potato masher or ricer. If you used a ricer, you’ll get an ultra-smooth result. Mix until the butter is fully absorbed.
Cream and season. Gradually stir in the warm milk or cream, mixing until the mash reaches your desired consistency. Season generously with salt and white pepper. Add a pinch of nutmeg if you like for that classic Scandinavian touch. Taste and adjust salt/pepper as needed.
Serve hot. Spoon the rotmos into a warmed serving bowl. (You can drizzle a little more melted butter over the top if you wish.) Serve it family-style with your main dish and other sides, letting guests help themselves.
