Recipe:

1. Whisk together the yeast and water in a small bowl.
2. In a large bowl, mix the flour, salt, yeast water and olive oil and shape into a ball.
3. Knead the dough on a floured work surface for 6-7 minutes. Use your hands to push and fold hand the dough until it's more springy. Shape the dough back into a ball and put it back into the bowl. Cover with a tea towel and leave to rise in a warm place until the dough has doubled in size (at least 45-50 minutes).
4. Top tip: you can use your oven as a proofing oven. Preheat the oven to 50C and place a baking dish filled with boiling water on the bottom rack. Turn off the oven and place the dough on the middle or top rack and shut the door. The steam and heat will create a warm and steamy environment for the dough, which will help it rise.
5. Preheat the oven to 230 C/210 fan/gas mark 5. Finely chop the celery and grate the mozzarella and feta cheese.
6. On a lightly floured surface, push or roll the dough out into a rough oval shape (about 25 cm long), then transfer it to a lined baking sheet.
7. Lay the grated cheeses evenly across the centre of the dough, leaving a 2cm border. Fold the edges over the cheese to enclose the cheese and make a "boat". 
8. Add the chopped celery on top of the cheese.
9. Put in the oven and bake for 15-20 minutes or until the crust is golden and the cheese is melted. Remove from the oven and add the egg yolk into the centre of the cheese bread. Serve warm.