1. Preheat oven to 350 degrees. Oil 2-quart baking dish or gratin. In medium saucepan or large, heavy skillet, heat 1 Tbsp of olive oil, and add spices. When they begin to sizzle, add cooked quinoa. Stir together for one minute, and remove from heat. Taste and adjust salt. Transfer to the baking dish, and spread in an even layer.
2. Arrange sliced beets over quinoa. Drizzle on remaining olive oil, cover and place in oven for 20 minutes or until hot. Meanwhile, place garlic in mortar and pestle with generous pinch of salt, and mash to a paste. Whisk or stir into the drained yogurt.
3. Remove the quinoa and beets from the oven, and top with dollops of yogurt. Sprinkle with walnuts, and serve.
Advance preparation: The cooked quinoa and the roasted beets will keep for three to four days in the refrigerator. You can assemble the casserole without the yogurt up to a day in advance. Cover tightly and refrigerate.