Combine the teff flour and 12 cups water. Mix, cover and store in a dark, cool place for 3 to 4 days to ferment, giving it its sour flavor. On the last day of fermentation, mix the teff flour mixture with the barley and wheat flours. Let rest for another 8 hours.
Heat a large cast-iron pan over medium heat. Pour 1 cup fermented mixture into it, swirling the pan to cover the entire surface area. Cover with a lid, 2 to 3 minutes. Injera is ready to plate when holes have formed on the surface.
This recipe was provided by a professional chef or restaurant and makes a large quantity. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.