- With the rack in the lowest position, preheat the oven to 400°F (200°C).
- In a food processor, combine the flour, baking powder and salt. Add the butter and pulse a few seconds at a time until it forms pea-sized pieces. Add the water and pulse again until the dough just begins to form. Remove the dough from the processor and form into a disc with your hands.
- On a lightly floured work surface, roll out the dough to line a 10-inch (25 cm) quiche plate. Fold in the excess dough, pressing well, to double the sides. Refrigerate for 30 minutes or freeze for 15 minutes.
- Meanwhile, in a skillet, soften the onions in the butter. Add the vinegar and cook for about 2 minutes or until completely dry. Let cool.
- In a bowl, beat the eggs with the milk, cream and thyme. Add the cooled onion mixture and stir. Season with salt and pepper. Pour into the crust. Bake for about 40 minutes or until the quiche is golden brown.
- Cut into wedges. Serve with the sour cream, smoked salmon and lettuce.