Place sunflower oil, salt and flour in a bowl. Combine milk and sugar and sprinkle yeast on top to dissolve. Pour into the flour mixture and knead with the dough hooks of a hand mixer for 5 minutes until dough no longer sticks to the side of the bowl. (Alternatively, knead vigorously by hand until the dough is smooth and elastic.) Cover the dough and leave it to rise in a warm place for 30 minutes.
Gently heat the bacon in a frying pan until the fat becomes transparent (a process known as rendering). Add the onion; fry until transparent. Whisk the sour cream, egg yolks, oil and nutmeg in a jug.
Brush a baking tray with oil. Knead the dough on a lightly floured work surface for about 5 minutes. With wet hands, stretch the dough out on the tray and shape the edges to form a narrow rim. Prick the surface of the dough with a fork.
Leave the dough to rise for a further 30 minutes until it is roughly doubled in size. Preheat oven to 220°C.
Spread the onion mixture, then the cream mixture over the pastry. Bake 15–18 minutes. Cut into slices and serve immediately.