1 Heat the sauté pan to 2/3 of the maximum power of your stove and test the heat using the ‘water drop’ technique.
2 Add the oil and butter to the pan, then add onions.
3 Brown the onions for 10 minutes, stirring regularly, then pour the balsamic vinegar and add sugar.
4 Cover and cook for 20 more minutes on medium heat. Season.
Preheat the oven to 375 °F.
5 Spread the puff pastry in the Demoul’vite tart mold and spread the sour cream over the pastry base.
6 Add the onions and sprinkle with diced Brie cheese.
7 Bake for approximately 30 minutes.