-Heat your oven to 375 degrees and butter a 2.5-quart oval gratin dish or 9-by-13-inch baking dish.
In a small sauce pan add the following:
1.5 cups heavy cream
3 sprigs thyme or sage
-Bring to just under a boil over medium low heat.
-Remove from heat and let stand 30 minutes.
-Strain through a fine sieve and discard solids.
-Peel 1 pound of potatoes and 1 rutabaga.
-Slice both as thinly as possible.
-Arrange the vegetable slices in an overlapping fashion in the baking dish, alternating them and seasoning with sea salt and freshly ground pepper as you go.
-Dot with 3 TBS butter.
-Pour cream over top (it should almost cover the vegetables; add up to 1/2 cup more if necessary) and cover tightly with parchment-lined foil.
-Bake until vegetables are tender, about 45 minutes, then remove foil and bake until golden brown and bubbling, about 15 minutes more.