Warm 300 ml of water lukewarm. Put flour and salt in a bowl. Make a depression in the middle. Crumble yeast into it, mix with some water, dust with flour from the edge. Cover and leave to rise in a warm place for about 15 minutes.
Add the rest of the water and knead everything with the dough hooks of the hand mixer to a smooth dough. Cover and leave to rise in a warm place for about 1 hour. In the meantime clean, wash and cut the leek into rings.
Dice the bacon and leave it crispy in a hot pan. Add butter and steam leek for about 3 minutes.
Coarsely grate the cheese. Wash sage, shake dry, pluck leaves from the stalks and chop. Knead yeast dough briefly. Roll out to a rectangle (approx. 25 x 50 cm) on a floured work surface.
Whisk the egg with a fork and brush the dough with it. Mix about 150 g cheese, sage and pepper and spread evenly on the dough, leek and bacon along one long side. Roll up the dough from the long side.
Place the roll of dough with the seam facing down on a baking tray lined with baking paper and form into a ring. Press the ends together. Brush with the rest of the egg and sprinkle with cheese. Leave to rise for about 30 minutes.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 30 minutes. Serve sprinkled with chive rolls.