1. For the crust: Cut the butter into small pieces and work it into the flour with your fingers. Add the salt and quickly gather the dough together. Cover and refrigerate for about 30 minutes.
2. Set aside approx. 50 g of the dough for decoration, then press the remainder into two greased loose-based mini-pie pans (approx. 15 cm in diameter). Press the dough all the way up the sides. Prick the crusts and pre-bake in the middle of the oven for approx. 12 min. at 400°F (conventional oven).
3. For the filling: Place the drained feta in a bowl and mash slightly with a fork. Add the eggs, lemon juice, thyme, salt and pepper and whisk until combined. Stir the finely grated beets into the egg mixture. Pour the mixture into the crusts.
4. Roll out the rest of the dough on a floured surface. Cut out hearts with a cookie cutter or a knife. Arrange the hearts on the tarts. Finish baking the tarts in the middle of the oven for approx. 35 min. at 400°F (conventional oven).
5. For the quick-pickled beets: Place water, lemon juice, sugar and peppercorns in a saucepan and bring to a boil. Add the beets and cook uncovered for about 15 min. over low heat. Drain the beets and let them steam off in a colander until serving.
6. Serve the warm tarts with a mixed green salad and the quick-pickled beets.