In the bowl of a stand mixer, combine milk, yeast, flour, sugar, butter, and salt, and knead until a soft dough forms, about 5 minutes. Transfer the dough to a lightly floured surface and knead with your hands for one more minute.
1 cup milk, 2 teaspoons instant dry yeast, 3 cups all-purpose flour, 1 teaspoon sugar, ½ teaspoon salt, 6 tablespoons unsalted butter
Transfer the dough to a lightly greased bowl, cover, and let the dough rest in a warm spot for 40 minutes or until about doubled in size.
Make the onion topping: In a large skillet, cook the bacon until crispy then add the onions. Reduce the heat to medium-low and let the onions slowly caramelize to a nice golden brown, about 20 minutes. Remove from heat and allow the mixture to cool for a few minutes.
5 slices bacon, 2 ½ lbs yellow onions
In a bowl, combine sour cream, eggs, salt, and caraway seeds. Add the cooled onion mixture to it and stir to combine.
¾ cup sour cream, 2 eggs, 1 teaspoon salt, 1 teaspoon caraway seeds
Grease an 18x13-inch baking sheet. With floured hands, spread the dough evenly about 1/2 inch high. Let it rise while you preheat the oven to 390℉ (200℃).
Spread the onion mixture evenly on top. Bake in the middle of the oven for 35-40 minutes, until the top is light brown and the filling is set. Let it cool for a few minutes before serving. This tart tastes best lukewarm.