Preheat oven to 400°
Fill a large pot with water and bring to a boil. In the meantime, toss the chopped carrots, parsnips, and rutabagas in a bowl with olive oil. Spread the vegetables onto a baking sheet, sprinkle with salt, and roast in the oven. Check the vegetables and stir them every 15 minutes, until starting to brown and easily pierced with a fork.
Scrub and quarter potatoes. Peeling is optional. Place in a large pot, cover with water, and bring to a boil. Boil the potatoes until easily pierced with a fork. Drain in a colander, return to the pot, and return to medium heat to dry out.
Remove from heat and mash the potatoes with a fork or masher, adding 2 tablespoons butter, 1 cup milk, and salt and pepper to taste. Set aside.
In the other pot of boiling water, add the cabbage and kale, along with a tablespoon of salt. Cook, uncovered, for 10 minutes, stirring occasionally. Drain.
Melt 2 tablespoons butter in a large skillet. Add the greens and cook for 10 minutes until slightly browned. Add the greens to the mashed potatoes and root vegetables.
Serve as is, or reheat in a 350° oven.