Preheat oven to 450 degrees F (230 degrees C).
Coat ten cups of a popover pan or ten 2 1/2-inch muffin cups with Nonstick Cooking Spray (as needed). Fill any empty cups with 1/2-inch of water so the pan does not burn.
In a large bowl, whisk together the All-Purpose Flour (1 cup) and Salt (1/2 teaspoon).
In a second bowl, whisk together the Large Egg (2) and Milk (1 cup). Add the egg mixture to the flour mixture and stir just until mixed.
Stir in the Shredded Extra Sharp Cheddar Cheese (1 cup) and Scallion (3) until combined.
Fill the prepared cups about three-quarters full with about 1/3 cup of the batter.
Bake for 15 minutes in preheated oven.
Reduce the oven temperature to 350 degrees F (180 degrees C) and bake an additional 10 to 15 minutes or until browned and puffed. Do not open oven door until the end of baking time or popovers will collapse.
Remove from the oven and immediately remove the popovers from the pans. Pierce the side of each popover with a knife to release steam. Serve immediately while warm.