Onion and Potato Tart

How to prepare onion and potato tart

1. Put the butter and water in the freezer for 15 minutes.
2. Boil the potatoes in their skins. Peel them and slice them.
3. In a food processor, put the flour, a pinch of salt, and then the cold butter cut into cubes. Pulse a few times until the flour becomes sandy. Then start the processor again and add water, one tablespoon at a time. The dough should come together into a ball in the processor.
4. With cold hands (hold them under cold running water) turn the dough onto plastic wrap, shape it into a disc and flatten it as much as you can. Refrigerate for at least 30 minutes.
5. Slice the onion into rounds (I also added a stray green onion from the fridge) and sauté it in a little oil (about 2 tablespoons) with salt, pepper, and a little thyme until very soft.
6. Beat the sour cream with the egg, a little salt, then fold in the grated cheese.
8. Grease a 22cm tart pan with a little oil.
   * my pan was 26cm diameter but the crust was just barely enough and I had to stretch it thin to cover the whole pan, and it showed when baked - the edges didn't come out perfectly; use a slightly smaller pan.
9. Roll out the crust slightly larger than the pan size, then place it inside, pressing with your fingers around the edge to fit into the grooves typical of tart pans. Roll the rolling pin across the edge of the pan to cut off the overhanging dough.
10. Spread the onion over the crust.
11. Arrange the potatoes over the onion.
12. Pour the sour cream mixture on top and sprinkle a tiny bit of thyme over it.
    * the assembled tart can be kept in the refrigerator until you want to bake it.
14. Bake at 220°C for 30 minutes, until the tart is golden and the crust is done.
15. To remove from the pan: place a plate of the same diameter on top of the tart, face down, and flip so the tart falls onto the plate crust-side up. Repeat with another plate to flip the tart crust-side down. Slice and serve warm.

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