Use a 28cm springform tin; grease the tin. Place flour in a bowl and make a well in the centre. Sprinkle yeast onto the milk. Pour into well; sprinkle with flour from the sides. Cover and let rise in a warm place for 15 minutes.
Add 1 tablespoon butter and the salt to yeast mixture. Knead into a dough. Cover and leave to rise in a warm place for 30 minutes. Knead vigorously for 10 minutes; leave to rise for a further 30 minutes.
Preheat oven to 220 degrees C. In a frying pan, heat remaining butter and fry the onion until transparent. Whisk the eggs in a bowl; stir in onions.
Roll out dough to line the tin. Pour in onion mixture; sprinkle with the bacon and caraway seeds. Bake for 10 minutes. Reduce oven temperature to 180 degrees C. Combine egg yolks, a pinch of salt and the cream. Pour mixture over tart; bake a further 50 minutes.