Preheat oven to 350°.
Spray or grease a two-quart casserole dish with cooking oil.
Beat eggs, yogurt, and salt together.
Melt butter and lightly saute onion. Add thawed potatoes. Stir to mix.
Pour in egg mixture and lightly stir to blend ingredients.
Pour into casserole dish. Sprinkle grated cheese over casserole.
Bake at 350° for approximately 25 minutes or until knife comes out clean when inserted.
Cut into 8 squares.