Preheat your oven to 350°F/175°C. Grease a standard loaf pan.
Combine all dry ingredients (both flours, sugar, baking powder, baking soda, and salt) in a large mixing bowl. Whisk together.
If you have a large enough measuring cup, combine all wet ingredients (starter, milk, egg, and oil) in that cup and whisk together.
Anything else you want to add in? Chopped fresh herbs are delicious (we like rosemary over here, because we have it growing in abundance in our garden). Shredded cheese, dried fruits, seeds, whatever suits your meal.
Gently fold the wet ingredients into the dry ingredients with a rubber spatula, stirring only enough to combine.
If you want to add anything on top, go for it! I sprinkled a handful of finely shredded grana padano on top for this version.
Bake in the middle of your oven for 45 minutes. When the timer goes off, check that it is baked through by the toothpick test. If the toppings are burning but the bread isn't quite done, place a sheet of aluminum foil over the top without crimping down the edges.
Remove from oven, and let cool for a few minutes in the pan before removing onto a rack to cool the rest of the way. It will get soggy if left in the pan too long.
I prefer this combination of all-purpose and whole wheat flour, but you could certainly use only all-purpose flour or experiment with other flours. I sometimes use whole wheat pastry flour in place of both the all-purpose and whole wheat, and it works well.
The weight of your sourdough starter will depend on when you last fed it. I've made this particular recipe with my starter at all different stages; the weight reported here was when it was about about 2 days out from being fed, so it was very dense and liquid. It's good regardless, just gives a more sour tang the "riper" it is (i.e. the further from the last feed).