1. Preheat oven to 425 degrees F. Cut the greens away from the beets, leaving about 1/4" of stems. Scrub the beets and place in a baking dish or lidded ovenproof casserole. Add 1/4" water to the dish. Cover tightly. Place in the oven and roast until fork-tender (30-60 minutes).
2. While the beets are in the oven, stir the garlic into the yogurt and refrigerate until ready to use.
3. Stem the greens and wash. Set aside.
4. Cool beets until safe to handle (15 minutes). Trim away stems and remove skins. Cut into wedges and set aside.
5. Heat oil in a large, wide skillet over high heat and saute the greens until just wilted (3-5 minutes). Season with pepper and lemon juice to taste.
6. Serve yogurt as a side dressing for the beets and greens, garnished with walnuts.