Dissolve yeast and sugar in 2 tablespoons of the lukewarm milk in a bowl. Let stand until yeast softens and begins to form a creamy foam, about 5 minutes.
Sift flour into large bowl, add yeast mixture, melted butter, remaining milk, egg, and salt. Stir well to combine until dough has pulled together. Turn dough out onto a lightly floured surface and knead until smooth and elastic, about 8 minutes.
Place dough in a large bowl. Cover with a towel and let rise in a warm place until doubled in volume, about 1 to 1 1/2 hours. Punch dough down, cover with a towel, and let rise in a warm place for 1 hour.
Place bacon pieces in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 5 minutes. Add onion and saute on low heat until soft, about 15 minutes. Remove from pan and cool slightly.
Mix bacon-onion mixture with sour cream and season with a little salt and pepper (be careful not to over-salt, as bacon is already salty).
Preheat oven to 400 degrees F (200 degrees C). Grease a 10- or 12-inch springform pan.
Roll dough on lightly floured surface to fit springform pan and cover pan. Fill with bacon-onion mixture.
Bake in preheated oven until dough is set and filling is slightly browned, about 30 minutes.