1. Infuse liquid: Heat oven to 375 degrees, and butter a 2 1/2-quart oval gratin dish or 9-by-13-inch baking dish. Bring cream and thyme to just under a boil in a small saucepan, then remove from heat and let stand 30 minutes. Strain through a fine sieve and discard solids.

2. Assemble: Arrange the vegetable slices in an overlapping fashion in the baking dish, alternating them and seasoning with salt and pepper as you go. Dot with butter, if using. 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.

3. Bake: Bake until vegetables are tender, about 45 minutes, then remove foil and bake until golden brown and bubbling, about 15 minutes more. Serve at once.