In a saucepan over medium heat, sauté onions in 1 Tablespoon margarine until softened.
Stir in salt, garlic powder, pepper, and flour until smooth. Add milk slowly, stirring rapidly to avoid lumps. Stir constantly until the sauce bubbles and thickens slightly. Remove from heat .
Place mixed potatoes and turnips in layers in a lightly oiled or sprayed baking dish.
Pour sauce over the potato mixture; sprinkle with parmesan cheese.
Melt 2 teaspoons butter; mix with the dry bread crumbs and sprinkle evenly over top.
Bake at 350 degrees for 50 minutes or until the sauce is bubbling and the bread crumbs have browned. Potatoes and turnips should be tender when poked with a fork.
