Mix the dough. Combine the flour, sourdough starter, sugar, and salt in a large mixing bowl. Gradually add the milk and mix until a soft dough forms. If the dough is too dry, add more milk until it comes together.
First rise. Cover the bowl with greased plastic wrap. Let the dough rise in a warm, draft-free place, such as a turned-off oven, for 8-12 hours or until the dough has doubled in size.
Prepare for cooking. After the first rise, sprinkle the baking soda over the dough. Then, knead the dough gently in the bowl for a few minutes to add the baking soda evenly.
Shape the muffins. Put the dough on a surface with a little flour on it. Next, roll the dough until it's about ½ inch thick. Use a 3-inch round cutter to make muffins. Take the leftover dough, roll it out again, and cut more muffins.
Second rise. Dust a baking sheet with cornmeal and place the cut-out muffins on it. Then, sprinkle more cornmeal on top of the muffins. Cover them with greased plastic wrap and let them rise for another 30 minutes. This second rise is crucial for developing the classic nooks and crannies in English muffins.
Cook. Heat a griddle or skillet over medium-low heat. If you like, sprinkle additional cornmeal on the cooking surface. Cook the muffins for 5-10 minutes on each side, or until they are golden brown and cooked through.
Cool and serve. Transfer the cooked muffins to a wire rack to cool. Once cool, split the muffins with a fork and toast them before serving. Enjoy with butter, jam, or your favorite toppings.