- Sift the flour into a large mixing bowl. Make a deep well in the middle. Crumble the yeast into the well. Add the sugar and some lukewarm water. Stirring in the well, incorporate some flour into the liquid. Cover the bowl and leave it in a warm place for about 15 minutes.
- Add the salt, oil, and 7fl oz of lukewarm water to the bowl. Using your fingers, mix to form a dough. Turn out the dough onto a floured work surface. Using your hands, knead vigorously. Cover and let rise for 1 hour.
- While the dough is rising, peel the onions and slice them into thin rings. Dice the bacon finely.
- Heat the butter in a frying pan, add the bacon, and sauté. Drain the bacon on paper towels. Sauté the onions in the bacon fat for about 5 minutes, remove them from the heat and let cool. Whisk together the sour cream and egg yolk, and season with salt, pepper, and nutmeg.
- Preheat the oven to 425°F. Line a baking tray with parchment paper. Knead the dough again. Divide it into eight portions. Roll out each portion into a thin oval tart.
- Place the tarts on the baking tray and spread them with sour cream. Place onions and bacon on top, followed by a sprinkling of caraway seeds. Bake the tarts in the middle of the oven for about 20 minutes until they are crisp.
Tip: To make a vegetarian version, top the sour cream with thinly sliced tomatoes and zucchini. Season with salt and pepper, sprinkle thyme and grated Gruyère over the top, and bake as above.