Start by making the dough. In a bowl of a stand mixer, put all the dough ingredients (if using fresh yeast, they need to be proofed) except the butter, mix briefly with a spoon. Then, using the hook attachment, knead the dough for about 5 minutes, then add the very soft butter, continue kneading for about 3-5 minutes.
Transfer the kneaded dough into a large bowl sprinkled with flour, cover with a clean cloth and leave to rise for about an hour. After this time, the dough should double in volume.
Rinse the lentils in a sieve and cook in salted water until tender, about 15 minutes. Drain and set aside to cool.
In the meantime, prepare the filling. Heat the oil in a frying pan, add the finely chopped onion and fry over low heat for a few minutes until soft. Then add the mushrooms (peeled or cleaned), cut into thin slices. Mushrooms will release a lot of water, fry them until most of the water has evaporated, add herbs de Provence, season with salt and pepper. Set aside to cool, then mix with lentils.
Preheat the oven to 180°C.
Divide the dough into smaller pieces. Put each of them on a floured countertop, roll it out to a thickness of about 0.5 cm. Using a glass or mug, cut out large circles. Put a portion of the filling and a little grated cheese, in the centre of each circle, form as in the case of traditional dumplings, sticking the edges together. Place the dumplings on a baking sheet lined with baking paper. Additionally, you can press the edges with a fork.
Place your dumplings aside for 15 minutes. After this time, brush them with the egg wash and sprinkle them with the nigella seeds. Bake for about 20-25 minutes, they should be browned but not too dry. The dumplings may seem hard right out of the oven, but they will soften slightly. Serve with borscht or broth. Enjoy your meal!