About 12 muffins or 2 to 2 ½ dozen mini muffins
Sift together the flour, sugar, baking powder, and salt in a medium bowl. Make a "well"/hole in the center of the mixture.
In a 2-cup measure, combine the egg, milk, and oil, blending well. Add egg mixture to dry ingredients all at once, mixing with a fork or spoon (but do not beat). The batter will contain some lumps. (Over-mixing causes tunnels in the muffins.) Fold in cheese and onion or green onion, if desired, mixing gently.
Spoon batter into 2 to 2 ½ -inch diameter or miniature muffin cups lightly coated with non-caloric vegetable cooking spray, filling each about two-thirds full. Bake in a preheated hot oven (400 degrees F.) for 20 to 25 minutes, or until muffins are golden brown and done. Serve hot or at room temperature. May store in an airtight container in a cool place. Muffins may also be wrapped in heavy-duty aluminum foil or placed in plastic freezer bags and frozen.
Variations: Add about ½ cup crumbled crisp-cooked bacon to batter with the cheese and onion.
Add ½ cup well drained crushed canned pineapple-in-its-own juice to batter with cheese. May omit onion, if desired.