Preheat the oven to 190℃/170℃ Fan/375℉, and grease a loose bottom 8-inch tart pan.
Sift the flour into a bowl and add the thyme and salt.
180 grams all purpose flour, a pinch dried thyme, 1 teaspoon salt
Using your fingers rub the butter into the flour till it is like breadcrumbs and add the water 1 tablespoon at a time to make a smooth dough (if it seems a little dry, add a few more drops of water). Roll into a ball, wrap and leave it to chill in the fridge for 30 minutes.
1 tablespoon butter, 3 tablespoons cold water
Unwrap and break off small pieces to line the bottom and side of the pan, pressing into the sides of the pan. Use a fork to make holes in the pastry, then line with parchment paper and cover with baking beans or raw rice. Bake in the oven for 10 minutes then remove paper and beans, reduce the temperature to 180℃/160℃ fan/350℉ and bake for a further 5 minutes.
Prepare the filling
In a skillet on low heat, melt butter and saute the sliced onion. Stir and cook till the onion turns golden, then add garlic and cook for a further minute (till the garlic turns a light golden). Remove from heat and set aside.
110 grams cold butter, 1 medium red onion, 1 clove garlic
In a medium mixing bowl, whisk together the creme fraîche, eggs, salt, mustard and crumble in the feta (keep some aside to crumble over the tart).
½ teaspoon salt, 180 grams creme fraîche, 1 teaspoon Dijon mustard, 2 medium eggs, 200 grams feta cheese
Heat the oven to 190℃/170℃ Fan/375℉.
Add the beets and onions to the pastry shell and pour the creme fraîche mixture over the top. Add the remaining feta and bake for 15 minutes, till the filling is set. Garnish with parsley and serve warm or at room temperature.
Prepare the filling while the dough is in the fridge.Season the onions with salt while they are cooking to speed up cooking time.To serve, sprinkle over some freshly chopped parsley, or a gremolata.Raw pastry dough will keep in the fridge for up to 2 days, or you can store it in the freezer for up to a month. Defrost in the fridge overnight before using.Wrap the tart in plastic wrap/wax paper and keep in the fridge for up to 3 days, or freeze for up to a month (for best flavour). Defrost in the fridge overnight and bring to room temperature before serving.