Instructions

1. Peel the onions and slice them thinly.
   
2. Heat a bit of olive oil in a pan over low heat. Add the onions and cook gently until soft, about 10–15 minutes. Once the onions are nearly cooked, stir in the balsamic vinegar, brown sugar, and thyme. Cook for a few more minutes until nicely caramelized. Remove from the heat and let cool.
   
3. While the onions cook, crumble the feta with your hands or finely chop it.
   
4. Once the onions have cooled, preheat the oven to 200°C/390°F (180°C/355°F if using a fan-assisted oven).
   
5. In a large bowl, beat the egg. Add the cream and a pinch of salt, and whisk until smooth.
   
6. Add most of the caramelized onions and feta to the egg mixture. Reserve a few onion slices and a bit of feta for decorating the top. Mix well.
   
7. Lightly grease your tart pan with olive oil. Line the bottom and sides with puff pastry. Prick the bottom several times with a fork.
   
8. Pour the filling into the tart shell. Decorate the top with the reserved onions and feta. Fold the pastry edges inward slightly to help contain the filling.
   
9. Bake for 20–30 minutes, or until the pastry is golden and the filling is set. Let the tart cool slightly before serving.