In a small saucepan, add the water, milk, and butter. Turn the heat on low and melt the butter. Once the butter has melted, remove it from the heat and let the mixture cool slightly. The mixture should cool to a little warmer than room temperature. Do not let the mixture boil.
While the milk mixture cools, in a large mixing bowl, add the sourdough starter and sugar.
Once the milk mixture has cooled slightly, pour it into the large bowl while mixing the sourdough starter and sugar into the milk mixture.
Add the flour and salt to the large mixing bowl. Mix it until a shaggy dough forms. Cover the bowl with a damp kitchen towel and let it rest for 30 minutes.
After 30 minutes, shape the dough into a smooth ball. To do this, pull one side of the dough and place it into the center of the dough ball. Rotate the bowl and keep doing that until a ball forms. Cover with the kitchen towel and let it rise for 8-10 hours. After 10 hours, place the bowl in the refrigerator overnight.
In the morning, turn the dough out onto a lightly floured surface. Let the dough rest for 10 minutes. Flatted the dough into an oval or rectangle. With a cookie cutter or a large cup, cut out 12-14 English muffins. Place them on a parchment lined baking sheet sprinkled with cornmeal. Let the English muffins rest for 1 hour.
After 1 hour, place a nonstick pan on the stove over low heat. Add 4-6 muffins to the pan depending on the size of your pan. Cover for 8-10 minutes.
Flip the English muffins over and cook for another 8-10 minutes on the other side. Remove the muffins from the pan and place them on a wire cooling rack. Continue with the rest of the muffins.