Preheat the oven to 400˚F. Rub the beets with olive oil and a little salt, wrap with aluminum foil and place in a baking dish. Cut off about a quarter inch from the top of the garlic head, leaving the cloves a little exposed. Wrap the trimmed garlic head in aluminum foil, too, and place that in the dish with the beets. Roast for about 45 minutes to one hour, or until the beets pierce easily when punctured with a fork or knife and the garlic head is soft. Remove, and set aside.
Meanwhile, heat 2 tablespoons of olive oil in a medium frying pan over medium heat and cook the chopped onions until translucent and lightly colored. About a minute before removing the onions from the heat, stir in half the cumin, cardamon, and turmeric and all the nutmeg.
In a separate small bowl, stir the remaining spices, a little salt, and a tablespoon of olive oil into the Meltemi Greek yogurt. Set aside.
When the beets are soft, remove from the oven and let cool slightly, until you can handle them to peel. Peel and coarsely chop them and transfer to the bowl of a food processor together with the cooked onion and spice mixture. Squeeze the roasted garlic cloves, one by one, into the bowl of the food processor, too. Add the toasted walnuts and pulse to combine.
Cook the pasta according to package directions in amply salted water. Drain, reserving about a cup of the pasta cooking liquid. Toss the pasta with 2 tablespoons of olive oil and 2 tablespoons of the spiced Greek yogurt.
Add a quarter cup of the reserved hot pasta water to the beet mixture in the food processor and process for a few seconds until the mixture is well combined and the walnuts ground to a coarse meal. Add a little more hot pasta water as needed to achieve the consistency you like.
Heat the pasta and beet sauce together in a large skillet with another tablespoon or two of olive oil, just to combine and heat through. Serve in individual plates, topped with the chopped thyme and pumpkin seeds, with the remaining spiced Greek yogurt on the side.
