1. Mix yeast and sugar with most of the water and leave in a warm place for 15 minutes till frothy.
2. Sift flour and salt into bowl, make a well in the centre and add yeast mixture, any remaining water and egg. Mix well to a dough, then turn on to floured surface and knead for 5 minutes until smooth and elastic. Place dough in a clean bowl, cover with a damp cloth and leave to rise in a warm place for 45 minutes - 1 hour, until doubled in size.
3. Preheat oven to 400°F (200°C).
4. Roll out dough on a floured surface and use to line a greased 10 inch loose-bottomed flan tin. Prove for 15 minutes.
5. Derind and chop bacon. Peel onions and cut into rings. Melt butter in a pan and fry onions for 5 minutes till golden. Slice the cheese.
6. Cover flan base with onions and bacon and arrange cheese slices on top. Bake for 30-35 minutes. Remove from tin and serve hot.