In a bowl, mix the sourdough starter, warm water, and flour. Stir until well combined. Cover and let it ferment at room temperature for about 8 hours or overnight.
After fermentation, mix in salt, baking soda, and sugar. The batter will become bubbly—this is what gives the crumpets their signature texture!
Heat a non-stick skillet over medium heat and grease it lightly with butter or oil. Place crumpet rings in the pan and pour batter into each ring until about ½-inch thick. Cook for 5-7 minutes until bubbles form on top.
Once bubbles appear and the top looks set, remove the rings and flip the crumpets. Cook for another 1-2 minutes until golden brown. Serve warm with butter, honey, or jam!