To begin, preheat your oven to 200°C (390°F). In a large bowl, combine the flour, salt, sugar, and instant yeast. Gradually add the lukewarm milk and vegetable oil. Mix thoroughly until a soft dough forms. Knead for 5-7 minutes until smooth and elastic. Cover with a clean kitchen towel and let rise in a warm place for about 30 minutes, or until doubled in size.

While the dough rises, prepare the filling. Melt the butter in a large skillet over medium heat. Add the diced bacon and cook until lightly crisp. Remove the bacon and set aside, leaving the rendered fat in the skillet.

In the same skillet, add the sliced onions to the bacon fat. Cook over medium heat, stirring occasionally, for 10-15 minutes, or until the onions are soft and golden. Sprinkle in the caraway seeds (if using) and season with a pinch of salt and black pepper. Set the onion mixture aside to cool slightly.

In a medium bowl, whisk together the sour cream and eggs. Season with a pinch of salt and a few cracks of black pepper. Combine this mixture with the cooled onions and bacon. Stir gently until evenly distributed.

Once the dough has risen, punch it down and roll it out on a lightly floured surface. Transfer the dough to a greased 9-inch tart pan or spring form pan, pressing it evenly into the base and up the sides. Trim any excess dough.

Pour the onion mixture over the dough, spreading it out evenly. Smooth the surface with a spatula to ensure even baking. Place the tart in the preheated oven on the middle rack.

Bake the Zwiebelkuchen for 30-35 minutes, or until the filling is set and the edges of the crust are golden brown. Check occasionally to prevent overbrowning; if necessary, cover the edges with aluminium foil.

Remove the tart from the oven and let it rest for 10 minutes before slicing. Serve warm, garnished with a sprinkle of fresh parsley if desired. Pair with a crisp white wine or a refreshing glass of German beer to complement the savoury flavours. Enjoy this traditional dish as a hearty appetizer or a comforting main course.