In a large bowl, combine the all-purpose flour, 3/4 cup of the starter, warm water, and salt. Whisk vigorously for 2 minutes. If using an electric beater, mix on medium speed for 1 minute. The batter should be a bit thinner than pancake batter.
Add the white sugar to the mixture and whisk for an additional 30 seconds (or 15 seconds on medium speed).
Cover the bowl with plastic wrap and place it in a draft free area and let is ferment for 8 hours at room temperature. Once the batter is done fermenting the surface should become foamy. This could happen before the 8 hours. The batter will not rise much but will increase in volume by about 10-15%.
After fermentation, add the baking powder and baking soda to the bowl. Whisk for 30 seconds to combine thoroughly.
Grease four rings (3 inches wide) with melted butter or vegetable oil.
Brush a non-stick skillet lightly with melted butter and place the rings in the skillet. Turn the stove to medium-high heat (or medium for stronger stoves) and bring it to temperature.
Pour a little less than 1/4 cup of batter into each ring, filling them about halfway. Adjust the amount if your rings are larger than 3 inches wide.
Cook for 45 seconds to 1 minute on medium-high heat until bubbles start to appear on the surface (but not popping much yet).
Lower the heat to medium and cook for another 1 1/2 minutes. Bubbles should start popping around the edges.
Reduce the heat further to medium-low/low and cook for an additional 2 1/2 to 4 minutes, until the surface is set and no more bubbles are popping. Use a toothpick to help pop any remaining bubbles.
Use tongs to carefully remove the rings. You might need to run a knife around the edges if they stick.
Flip the crumpets and cook the other side for 20 to 30 seconds to add a bit of color.
Transfer the crumpets to a wire rack, golden side down, and let them cool completely.