Dissolve the fresh yeast in the water. Stir in bread and rye flours until well combined. Cover and leave it at room temperature for 2 hours, then transfer to the fridge for 12 hours.
Place all the dough ingredients together with starter in the mixing bowl of your stand mixer. Stir at low speed until all ingredients have been incorporated, then increase the speed and mix until the dough gathers into a ball. Add more water, a tablespoon at a time, if too stiff.
Cover with a plastic and set aside until it doubles in size, 1-2 hours. Turn out the dough onto a lightly floured work surface. Lightly press down the dough and shape into a round. Divide the dough into 10-15 pieces. Shape each piece of dough into a round or an oval. Place them on a baking sheet lined with parchment paper. Cover and proof until double in bulk. This will take about 90 minutes.
Preheat oven to 230C/450F and place an old baking pan on the bottom rack. Combine egg white and water in a bowl. Brush the top of bread rolls with egg wash and sprinkle with the poppy seeds. Slash a 1/4-inch/5-mm deep cut across the top of each roll. Place rolls in the oven on the next shelf up, pour 1 cup of water into the old baking pan and close the door quickly. Bake for about 20 minutes until richly brown.