Peel potatoes and slice. Layer in lightly greased glass baking dish.
Heat 3 Tbsp butter or margarine in saucepan over low heat until melted. Blend in flour and seasoning. Cook over low heat, stirring constantly until mixture is smooth and bubbly; remove from heat. Stir in yogurt and whipping cream. Heat to boiling for one minute, stirring constantly. Stir in onions.
Pour mixture over potatoes. Lightly stir the mixture and potatoes together, making sure the potato mixture is spread out evenly in baking dish.
Cover and cook in 350°F oven for 30 minutes. Uncover and cook until potatoes are tender, about 60-70 minutes longer. Let stand 5-10 minutes before serving.