In a medium bowl, stir together the flour and garlic salt. Mix in egg and water until smooth.
Heat the oil in a large deep skillet over medium-high heat. Test to see if oil is hot by frying a drop of batter. If it sizzles and floats to the top, the oil is ready.
Dip mushrooms into the batter and then place in the hot oil. Fry a few at a time so they are not crowded, until golden brown, 3 to 5 minutes. Remove from oil with a slotted spoon and drain on paper towels.