Sift flour into a mixing bowl, and make a little well in the middle. Pour ⅔ of the warm milk, a spoon of sugar, and yeast in it. Mix the middle with a fork, add some flour from the sides. Cover with a kitchen towel and let it rise for 30 minutes in a warm place.
Melt the butter in a small pot, add it to a bowl with flour and yeast (the butter must be warm, not hot!).
Add the rest of the lukewarm milk, sugar, an egg yolk a pinch of salt, and grated lemon zest.
First, mix the dough with a fork, then knead with your hands or in a kitchen robot for about 10 minutes. The dough should be non-sticky and smooth. Add some flour if the dough is too sticky while kneading.
Split the finished dough into two buns, cover with a clean towel and let it rise for 1,5 hours in a warm place.
Meanwhile, prepare the streusel – mix the sugar, flour, and butter and form the streusel using your fingers.
Place the leavened bun on baking paper and roll it out into a circle about 12 inches (30 cm) in diameter. If the dough sticks, you can sprinkle the baking paper with a little flour.
Around the edges, make a small rim and roll it in. You will use up about ½-1 inch (1-2.5 cm) of the dough edge.
Brush the edge of the dough with a beaten egg.
Place blueberries evenly on the dough, and sweeten them with sugar to your liking. How much sugar you pour over fruit depends on the sourness of blueberries. Usually, two tablespoons of granulated sugar per frgal is enough.
Sprinkle with streusel liberally and let the frgál rise for 15 minutes.
Transfer the frgál with baking paper carefully on a baking tray and bake it in the preheated oven to 340 °C (170 °C) for 20 minutes.
Melt the butter for final splashing in a pot and mix it with rum.
Once you take the frgál out of the oven, splash its surface with the butter & rum mixture.
Bake the 2nd frgál as described above.