1. Preheat the oven to 350 degrees. Oil a 2-quart baking dish or gratin. In a medium saucepan or a large, heavy skillet, heat 1 tablespoon of the olive oil, and add the spices. When they begin to sizzle, add the cooked quinoa. Stir together for one minute, and remove from the heat. Taste and adjust salt. Transfer to the baking dish, and spread in an even layer.
2. Arrange the sliced beets over the quinoa. Drizzle on the remaining olive oil, cover and place in the oven for 20 minutes or until hot. Meanwhile, place the garlic in a mortar and pestle with a 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 the 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.