In a large mixing bowl, whisk together all dry ingredients.
Combine water and sourdough starter in another bowl and mix well. Pour the mixture into the dry ingredients mixture. Use a clean hand to knead the dough. It should form a soft dough.
Let the dough ferment at 70-76 ℉ for 12 -16 hours. After the fermentation, the dough should be softer and have somewhat of a sour smell.
Preheat oven to 450 ℉.
Mix baking soda and water in a small bowl. Add the mixture into the fermented dough and knead to mix well. This will neutralize the acid and take away some of the sour taste of the dough.
Lightly grease a loaf pan (I use a glass 9” x 5” x 3.5” loaf pan) and transfer the dough into the pan. Smooth the surface of the dough with a damp hand. Insert a knife on top of the dough ½ inch deep and cut the dough lengthwise so the hot steam will escape from the gap instead of creating random cracks during baking.
Spray the surface of the dough with canola oil and cover the pan with aluminum foil. Place the pan on the middle rack of the oven. Reduce the temperature to 400 ℉. Bake with the foil on for 20 minutes. Remove the foil and bake another 40 minutes until the bread is golden brown.
Remove the bread from oven and transfer the bread onto a cooling rack to completely cool before cutting.
This recipe may work with other kinds of all-purpose gluten-free flour mix. You may need to adjust the amount of water added into the dough.
The first batch of sourdough starter may not turn the fermented dough very sour, so use only ½ teaspoon of baking soda when using the first batch of sourdough starter.