For the dough, put 250 ml of lukewarm water in a small bowl, mix with the yeast and sugar and leave to stand in a warm place for 5 minutes until foam forms. Add olive oil and mix everything together.
Sieve the flour into a mixing bowl and mix with the salt. Pour the yeast mixture into the flour and knead it into a dough with your hands. Dust a clean work surface with a little flour and knead the dough with your wrists for 5 minutes until it is soft and elastic.
Put the dough back into the mixing bowl, cover with a kitchen towel and leave to rise in a warm place for 45-60 minutes until it has doubled in volume, dust the work surface with a little flour and knead the dough with your fist to remove air pockets.
Divide the dough into 4 portion pieces. Roll individual portions into balls. Leave to rise for another 15 minutes in a warm place before use. Wrap unused balls in foil and freeze.
Skin and bone the trout fillets. Cut the trout meat into bite-sized pieces and set aside.
Put a pizza stone* in the oven and turn it on at full power. After the oven has reached its maximum temperature, preheat it for another 15 minutes.
Dust a clean work surface with flour, roll out the dough ball into a 30 cm Ø pizza base, place on baking paper and prick several times with a fork.
Mix the crème fraîche in a small bowl with mustard, some sea salt and freshly ground black pepper. Spread the crème fraîche mixture on the pizza base and sprinkle half of the chives, mozzarella and buffalo mozzarella over it.
Place the pizza on the hot pizza stone and bake for 5-10 minutes until golden brown. In the meantime, heat the butter in a coated pan and fry the potato slices in it at medium heat until golden brown, salt and pepper.
At the end of the baking time, carefully place the pizza on a cutting board or pizza plate with a pizza shovel or a wide spatula. Cover the pizza with the potatoes, apple slices and trout meat, drizzle with lemon juice and olive oil and garnish with watercress and the remaining chives.