Place beets in a large saucepan and cover with water. Simmer about 30-45 minutes until tender at knifepoint. Cooking time will vary depending on size of beets. Drain and set aside until cool enough to handle. Rub off skins, trim off any woody parts, and slice thin. Reserve. Meanwhile, if using regular spinach, tear leaves into bite size pieces and remove thick stems. Lay out spinach on a platter and reserve. In a small bowl whisk together garlic, olive oil, vinegar, nutritional yeast, salt and pepper. Pour into sliced beets and toss to mix. This can marinate up to several hours if you like. Place dressed beets over spinach, then sprinkle chopped walnuts over them. Garnish with dollops of yogurt, if desired.