Put flour in a bowl. Make a well in the center, place the yeast in it, and sprinkle sugar over it. Warm milk to about 35°C / 95°F, pour it into the well, and stir to activate the yeast. Let steep for 10 minutes. Add butter and egg yolk, then knead for 5 minutes until smooth. Cover and let rise for 30 minutes until doubled in size.
Peel and dice the 2 onions. Sauté the onions in a pan over medium heat for about 1 minute. Add the minced meat and cook for 3-4 minutes, breaking it apart with a spoon. Pour in the canned tomatoes and mash them. Season with paprika powder, ketchup, salt, and pepper to taste. Let the mixture cool to room temperature.
Knead the risen dough for 1 minute. Press it into a tart pan. Brush the cooled meat mixture evenly over the dough. Bake in a preheated oven at 180°C / 356°F for 20 minutes. Sprinkle grated cheese over the top and bake for another 5-10 minutes until the cheese is golden and melted. Remove from oven and let cool for 5 minutes.