Preheat oven to 400 degrees F. In a large bowl, combine the flours, baking powder, onion, salt, sugar substitute, and Italian herb seasoning. Make a well in the center.
In a separate bowl, mix the milk, margarine, and eggs.
Pour liquid mix into the well in the dry ingredients. Stir to combine until the batter is moistened
Spoon the batter into a 1 1/2-quart casserole dish coated with vegetable cooking spray. Sprinkle Parmesan cheese over the top and then bake for 25 to 30 minutes, or until a wooden toothpick inserted into the center comes out clean.
Remove from the oven and allow the loaf to cool in the dish for 10 minutes. Afterwards, remove the loaf from the pan and allow to cool completely on a wire rack.