ZWIEBELKUCHEN (SAVORY BACON-AND-ONION TART)

Steps:

• Place milk and 4 tablespoons butter in a small saucepan, and heat until butter has melted. Remove pan from heat, and let mixture cool until tepid.
• Place milk mixture in the bowl of an electric mixer fitted with the paddle attachment. On low speed, add the sugar, 1/2 teaspoon salt, lemon zest, egg yolks, and yeast. Gradually add 3 cups flour, creating a dough that is soft but not sticky.
• Turn dough out onto a floured surface, and knead until it is smooth and elastic and springs back when you poke it with your finger, about 5 minutes. Place in a large buttered bowl, and cover tightly with plastic wrap. Set in a warm place until dough has doubled in bulk, about 1 hour.
• Heat oven to 400 degrees. Line an 11-by-17-inch baking sheet with parchment. Punch dough down, and roll out to a 1/4-inch thickness. Place on prepared baking sheet, and cover with buttered plastic wrap; let rise until dough doesn't spring back when you poke it with your finger, about 20 minutes.
• Melt remaining tablespoon butter in a large skillet over medium heat. Add the bacon, and cook until golden and crisp and most of fat has been rendered, about 6 minutes. Add onions, and cook, stirring occasionally, until translucent, 8 minutes. Reduce heat to low. Sprinkle 1 1/2 teaspoons flour over onions, and stir in. In a small bowl, whisk together sour cream and the egg, and stir into the onion mixture. Cook, stirring, until mixture thickens, about 1 minute. Remove skillet from heat, and season with salt.
• Spread mixture on top of dough, and let rise 15 minutes more. Sprinkle top with caraway seeds. Bake until edges are crisp and brown, about 30 minutes. Transfer to a wire rack to cool, and serve. Bread will keep, wrapped and unrefrigerated, up to 4 days.