Preheat oven 180 degrees
Slice onions and sauté in pan until sweating and soft, add brown sugar and balsamic vinegar. Stir frequently over low heat until caramelised for about 15 minutes.
Dice bacon and fry until soft.
Line dish with baking paper, fit puff pastry in dish, line with more paper and blind bake with rice or ceramic beads until almost cooked. (So your puff is covered by paper before you add your rice or beads.) remove beads and paper.
Layer spinach, crumbled feta, onions and bacon in your almost cooked crust.
In a Separate bowl whisk eggs, sour cream, salt & pepper and milk. Gently pour into your dish and lightly press down filling.
Bake for about 30 mins until golden brown on top and until set.