1. Preheat the oven to 350℉.2. In a pot of cold salted water, cook the beets. Bring to a boil. Cover and cook for about 35 minutes. Pierce the tip of a knife to the heart to check cooking.3. Drain and peel the beets. Slice into thin rounds.4. In a gratin dish, drizzle with a little oil. Arrange the beet slices, overlapping them. Top with a layer of zucchini slices.5. Pour in the sour cream. Season with salt and pepper.6. Bake for 20 minutes.7. Remove from the oven and serve immediately.