Mix flour, water, salt, sourdough and the yeast to a homogeneous dough. It will be a really wet dough, with no structure, and it will fall to pieces, but that’s normal. Cover the mixing bowl and let it rest for half an hour in a warm place. After the resting phase, put some olive oil on the dough and on your hands and start to fold the dough from outside to the inside. At the beginning, it will be really hard, but it will get better. Repeat this process 4 or 5 times: 30 minutes rest, some olive oil and folds. When the dough gets some consistency, you can put it in the refrigerator for 24 hours.
The next day, oil a baking tray and tip over the focaccia. Spread the dough slowly and softly and let it rise another 2 hours somewhere warm. Wet the surface every now and then. By doing that, you prevent that the dough does not dry out. Preheat the oven on 250°C (convection oven/top and bottom heat). Before baking, add some water, oil and salt on the surface and press the dough with all your fingers, to make the classic focaccia holes. Bake 30 minutes till you have a nice brown crust. Cut the focaccia in 10 x 15 cm portions. To fill the focaccia, cut it in the middle and fill it with the buffalo mozzarella, the Speck, the apple slices, the pistachios and the dried tomatoes. Put some olive oil on top and serve.