Pour sourdough starter into a large glass or plastic bowl. Whisk in milk vigorously until frothy. Stir in eggs, butter, oil, sugar, flours and salt. DO NOT add baking powder and soda yet. Cover and refrigerate batter overnight.

In the morning, heat griddle or a large non-stick skillet over medium heat. Add oil to griddle or skillet and wipe most of it away with a bunched up paper towel.

Immediately before you're ready to cook pancakes, stir baking powder and baking soda into batter. It will become bubbly. Pour small amounts (1/4 cup to 1/3 cup depending on how large you want your pancakes) of batter into skillet using a ladle or small measuring cup. Make sure you don't place batter too close together or pancakes will run together and become difficult to flip. Drop desired amount of blueberries on top of the batter. Cook until batter starts to become firm at the edges, bubbles rise to the top of the batter and the bottom is golden, 3-5 minutes. Reduce heat to medium-low if pancakes brown too quickly. Flip pancake and continue to cook until bottom is golden brown and pancake is cooked through, about 3 more minutes.

Serve immediately with maple syrup or keep warm in a 200°F oven. Pancakes can be completely cooled and frozen in a freezer bag. Reheat in microwave.