Make the loaf

• Stir the yeast and sugar into the lukewarm water.
• Using the dough hooks, mix 200 g of the flour with the yeast mixture and kefir in a mixing bowl.
• Using the dough hooks, mix in the remaining 200 g flour, salt, egg white and 5 tbsp oil until the dough is smooth and elastic. It will still be very soft and sticky.
• Knead the dough briefly on a lightly floured work surface, then move it to a lightly oiled area and knead for 2 minutes.
• Press the dough into a lightly oiled 26 cm round pan, spreading it evenly all the way to the edges.
• Cover the pan with plastic wrap and let the dough rise for about 30 minutes, or longer if needed, until visibly puffed.
• Preheat the oven to 180°C (350°F) while the dough rises.
• Remove the plastic wrap. With a sharp knife, score a shallow grid of lines across the top about 4 cm apart, then brush the dough with the egg yolk beaten with the milk.
• Bake for about 35 minutes, until the top is firm, the top, sides and base are golden brown, and the centre reaches about 88°C (190°F). Continue baking if the centre is not ready; loosely cover the top with foil if it is already well browned.
• Turn the loaf out immediately and wrap it in a clean kitchen towel to cool for at least 30 minutes. Give it longer if needed to cool completely before tearing it into pieces.
Expect a very soft, sticky dough. Dust the work surface lightly, then finish kneading on the oiled area; adding flour until the dough feels dry can make the loaf dense.
Watch the dough as well as the clock. A cool room slows the rise, so give it more time if it has not visibly puffed.