Dissolve the yeast in lukewarm milk and leave for 10 minutes until a yeast foam forms on the milk.
Sieve the flour and mix with the salt.
Wash the lemon well, grate the zest and add to the flour.
With a dough hook: Add the yeast-milk mixture to the flour and knead.
Then add the egg and gradually the butter. Knead the dough for 20 minutes with the dough hook until smooth.
Grease a large bowl with a little oil and let the kneaded dough rise in it, covered, for about 4 hours in a warm place.
Prepare filling
Roast the pine nuts in a pan without fat and set aside.
Crumble the feta. Drain the tomatoes and cut into strips.
Mix the feta and tomatoes in a bowl with the olive oil and za'atar. Season with salt and pepper.
Fill and knot the babka
Turn the dough out onto a lightly floured work surface and roll out into a rectangle about the thickness of a finger.
Spread the feta mixture on the rectangle about 5 cm from all edges and scatter the pine nuts on top.
Now carefully roll up the rectangle from the longer side. The result is a filled babka snake.
Cut the babka snake lengthwise in the middle with a sharp knife. You now have two long pieces of dough, each with a cut surface where you can see the filling.
Place the dough pieces crosswise with the cut surface facing upwards and place the dough pieces on top and bottom one after the other until a plait is formed.
Grease the loaf tin with a little oil and place the plait in it. The plait will probably be longer than the tin. This is normal. Carefully compress the plait into the mould.
Leave the babka plait to rise for a further 90 minutes in the loaf tin and covered with a kitchen towel.
Preheat the oven to 190 °C top/bottom heat.
Bake the babka for about 45 minutes until golden brown and crispy.
Allow the babka to cool completely and then turn it out of the mould.