an instant-read thermometer and four 3-by-1-inch ring molds
In a large bowl, whisk together the flour, sugar and salt to combine.
Combine the milk and 1/2 cup warm water (about 100 degrees F) in a bowl and sprinkle with the yeast. Once the yeast starts to bubble, after 3 to 5 minutes, pour the liquid into the dry ingredients and stir with a wooden spoon to combine. The batter will be thick. Cover the bowl with plastic wrap and let it rise in a warm place for 30 minutes. The mixture will rise slightly and some bubbles will form on top.
Stir the baking soda and 1/4 cup warm water (about 100 degrees F) into the batter. Cover back up and let rest until doubled in size, another 30 minutes. Spoon the mixture into a large liquid measuring cup with a pouring spout for ease of pouring into the crumpet molds.
Put a cast-iron skillet over low to medium-low heat and spray with the cooking spray. Spray four 3-by-1-inch ring molds with the cooking spray and place in the hot skillet. Fill each ring half full with batter (the batter should spread inside the rings; if not, add another 1/4 cup water and stir well to thin it). Wait for bubbles to start to form, then top with some pecans and sprinkle with sugar. The bubbles will help keep the pecans and sugar from sinking to the bottom of the crumpets. Cook until golden on the bottom, 6 to 8 minutes. Remove the rings, flip and cook another 1 to 2 minutes, just to cook the other side. Repeat with the remaining batter.