Topping: Melt margarine and pour into 13x9 cake pan. Add the brown sugar and spread evenly on bottom of pan.
Peel, core and slice apples, 1/4 to 1/2 inch thick. Lay the apple slices in an overlapping pattern on the bottom of the pan.
If you like, you can sprinkle cinnamon on the apples.
Mix flour, baking powder and salt.
Cream margarine, yogurt and sugar together. If you want some added sweetness, you could try adding a little stevia.
Add the egg and vanilla and beat well.
Add this mixture and the milk to the dry ingredients and whisk until smooth. You can add cinnamon, ginger and nutmeg to the batter if you like (I use 1/2 teaspoon of each).
Pour batter evenly over the apples.
Bake for about 40 minutes at 350 degrees.
Loosen edges of cake away from the sides of the pan with a spatula and then turn cake upside down onto a platter. Serve warm.