Place a 12-muffin pan into the oven, and preheat to 450° F.
Using a microwave, or a small saucepan over low heat, warm the milk until it feels just barely warm to the touch (less than 30 seconds in my microwave).
Meanwhile, whisk the eggs and sourdough together in a bowl - note that it'll be easier if the bowl has a spout. Combine the warmed milk with the egg and sourdough mixture, then add the salt, and then finally the flour. Lumps are okay, but the batter should be relatively thin.
Carefully remove the pan from the oven, and spray it with non-stick spray. Pour batter about halfway up 6 of the cups - ensure you're staggering with empty cups as you go along to provide room for the popovers to grow. When you've filled the cups halfway, carefully move the pan back to the oven rack, then fill the cups to the top.
Bake the popovers for 15 minutes at 450 degrees, then reduce the oven temperature to 375 degrees. Bake for 15-20 minutes, or until the popovers are golden brown. Enjoy!