Preheat the oven at 180 degrees Celsius.
Thaw 2 sheets of ready made puff pastry on a flat surface. Prepare your ovenproof dish by spraying with a little olive oil.
Finely chop your onion and bacon and pop into a frypan and cook till softened and a tiny bit brown. Set aside and allow to cool.
Prepare your spinach by cutting out the stalk. (if needed). If you're using baby spinach you can miss this step.
Lay one sheet of the puff pastry over your prepared ovenproof dish making sure the pastry sides are sitting half way up. A bit like a square bowl of pastry inside dish. Then layer it with half of the spinach.
Whisk the eggs and sour cream (if you are adding it) and then mix in the grated cheese, salt and pepper and cooled fried bacon and onion mix and pour gently over the layer of spinach.
Add the rest of the spinach on top of the egg mix and then place the 2nd sheet of puff pastry over the top and tuck into the sides making sure the egg mixture doesn't ooze out.
Cut the tiniest slot on the top of the pie with a sharp knief and glaze with milk or egg wash and pop into the oven for around 45 to 55 minutes till golden brown. Allow to cool on a wire rack a little before slicing.