Place potatoes into a large pot and cover with water.
Bring to a boil, then reduce heat, cover, and cook for about 15-20 minutes, or until potatoes are tender. Drain.
Preheat oven to 350°F and lightly grease a 2 quart casserole dish with non-stick cooking spray. Set aside.
Pour potatoes into a large bowl. Add yogurt, milk, butter, salt, pepper, sage, and cheese. Mash until reaches your desired consistency.
Spread mixture into dish and sprinkle with additional cheese, if desired.
Bake for 30-35 minutes, or until mixture is heated through.
Serve immediately.