Preheat the oven to 180℃
Heat oil in a non-stick pan, add onions, and cook for 15 minutes or till very soft
Grease a 20cm round springform tin will oil
Arrange pastries on a flat surface, alternating with oiling them
Layer the pastries over one another at different angles
Carefully place the pastry structure into the tin and fold any hanging ends into the tin
Place the tin on a baking tray
In a separate heat-proof bowl, place the spinach and add boiling water
Drain the hot water and rinse with cold water, repeat
Pat the spinach dry with a paper towel
Arrange the onions, spinach, and grated cheese evenly on the pastry
Whisk the eggs and milk in a medium-sized bowl, season with salt, pepper, and garlic
Pour the eggs and milk mixture over the filling
Place the tin in the oven
Bake for 40 to 45 minutes or if a knife inserted into the center comes out clean
Remove the tin from the oven and let it cool for 5 minutes
Remove the quiche from the tin and cut it into wedges
You can serve the quiche warm or cold