Prepare the Starter: On day 1, stir together water and teff flour in a 16-ounce glass jar until smooth and combined. Wipe sides of jar clean. Cover with a loose fitting lid, and store at room temperature (about 65°F to 70°F) for 3 days.
Prepare the Injera: On day 4, stir 1 cup teff flour, 1 cup tap water, and yeast into jar with Starter; stir until smooth and no dry flour remains. Cover with a loose fitting lid, and let ferment at room temperature until almost doubled in size, 2 to 4 hours.
Heat a dry nonstick 10-inch skillet over medium-high. Stir sparkling water, 1 tablespoon at a time, into Injera batter in jar until you reach a crêpe batter-like consistency. Pour ¼ cup batter into preheated skillet, quickly swirling to evenly coat bottom of skillet. Wait for a few bubbles (eyes) to form on surface of Injera, 10 to 15 seconds; cover and cook until steam accumulates on lid and Injera looks cooked through, about 1 minute. Remove lid, and cook until edges of Injera begin to curve up, and Injera easily slides off skillet, 30 to 45 seconds. Transfer Injera to a parchment paper-lined sheet pan. Wipe skillet and lid clean. Repeat process with remaining batter.
