Preheat the oven to 375ºF. Spray a large casserole dish or baking sheet with nonstick cooking spray. Set aside.
Prepare the sweet potatoes and garlic. Cut off each end of the potatoes, then slice them in half long-ways (hot-dog style). Cut off the pointy end of the head of garlic to expose the cloves.
Place the sweet potatoes and garlic head on prepared baking pan, cut-side up. Drizzle with olive oil and season with 1/8 teaspoon salt and 1/8 teaspoon pepper.
Roast in the oven for for 20 minutes. Then, flip the sweet potatoes and garlic so they are skin-side up. Roast for another 20 minutes or until the sweet potatoes can be easily pierced with a fork.
Remove from oven and let cool for 10 minutes.
Use a spoon to scoop out the flesh of the sweet potatoes and place the flesh into a large bowl. Skip this step if you want mashed sweet potatoes with the skin on.
Squeeze the roasted garlic cloves out of their casing and add to the bowl with the sweet potatoes. Add the yogurt, butter, and remaining salt/pepper.
Use a hand mixer or immersion blender to mix everything together until smooth. Garnish with fresh thyme and serve topped with additional butter.