In a bowl, pour the flour, add salt and sugar, and mix well. Create a well in the middle, where you crack the egg. Pour approximately 1/3 of the milk and the oil into the well.
Start mixing the ingredients in the well with a hand whisk or a mixer while gradually incorporating the flour from around the well. Mix until you obtain a smooth and thick mixture without lumps. If necessary, you can add a little more milk.
Then, while continuously stirring, gradually pour in the remaining milk in a thin stream. When you've poured in all the milk and obtained a smooth, liquid batter, stop stirring. The batter should be thin but not too runny. If it's too runny, add a bit more flour.
Let the prepared batter rest for 15 to 30 minutes before cooking to allow the starch granules to absorb the liquid. Before using it for baking, give it a good stir once again.