Add the teff flour, all-purpose flour and yeast together in a bowl. Add 3 cups of warm water (105 ˚F) to the mixture and whisk until smooth. Cover the bowl with a lid and let it sit in a dry area for 2 to 3 days to ferment the batter.
To make the injera, start by carefully pouring off and discarding the water that has risen to the top of the batter. Add the salt to the batter and use a spoon to mix the batter.
Boil 1 cup of water in a small pot with the stove set at medium heat. Pour ½ cup of the batter into the boiling water. Let the batter cook for 1 to 2 minutes; stir vigorously as it cooks and make sure that there are no lumps in the cooked batter.
Let the cooked batter cool to room temperature. Use a spoon to scoop it into the uncooked batter and mix until smooth.
The batter should be about the consistency of crepe batter. I added ¼ cup of water to the batter to get it to the right consistency. You may need to add more or less water, depending on how much water you poured off the batter.
Let the batter sit with the bowl covered for 30 minutes to 1 hour or until a lot of bubbles form at the top of the batter.
Heat a crepe/flatbread pan on the stove at medium heat. Pour ½ cup of the batter in a circular motion onto the pan, starting at the center of the pan and working your way outwards.
Cook the batter until the edges become firm and a lot of bubbles form at the top (about 30 seconds).
Cover the pan with a lid and let the batter cook for about 45 seconds; I just use aluminum foil to make a dome lid over the pan, making sure that it doesn't touch the batter.
When it's ready the injera will be firm throughout and bubbles will be at the top of it.
Carefully slide the cooked injera off the pan and onto parchment paper to cool down.
Repeat the steps with the remaining batter.