Heat oil in large pan and cook onion for about 7 minutes.
Add garlic and cook for a further minute.
Add spinach and cook until spinach has wilted.
Remove from heat and drain mixture in a colander, pressing with the back of a spoon to remove excess moisture.
Combine the egg and cheeses and add to spinach.
Lightly spray a lamington tin with cooking spray.
Place 6 phyllo sheets on the base of the tin, lightly buttering between each layer and adding cheese.
Spread the spinach and cheese mix evenly over the phyllo and then add the remaining 6 buttered sheets of phyllo, adding cheese except for the last 2 layers.
Using a sharp knife cut the spanakopita into squares.
Sprinkle the top with 1-2T water and cook in a moderate oven for 30-40 min or until golden.