Peel the rutabaga - this is easier with one of the Y-shaped peelers [find a link below post], than with a regular side peeler. Cut the rutabaga into 1 inch cubes.

Put the cut rutabaga in a 3 quart sauce pan, add a generous couple pinches of salt, fill with water to nearly cover the cubes, and set over medium heat, covered. When it comes up to a simmer, cook for about 15 minutes.

Then, if you are using it, add the scrubbed and cubed red potato. [it adds a little body to the final dish, but can be left out if you don't want the added carbs], and bring back up a simmer.

Cook for another 15 minutes or so, until the rutabaga is cooked through - a small knife inserted into a cube should meet no resistance.

Meanwhile, melt the butter in a small saute pan, and add the minced garlic and the onion slices, and sprinkle lightly with about 1/4 teaspoon kosher salt. Keep the heat low, so that they become soft without getting much, if any, color on them.

When the rutabaga is done, completely drain off the water. I like to put the pan back over medium heat, and stir the contents around a bit, to evaporate the excess water, just like when making mashed potatoes, but it's optional.

Mash the rutabaga roughly - it should not be smooth, but rather kind of a rustic texture.

Fold in the onions, garlic, cheese and buttermilk. Taste, and add additional salt and pepper, if needed. Serve hot, and refrigerate any leftovers.