Whisk the yeast into 300ml of lukewarm water, leave for 2 minutes, then pour into a large bowl with 500g of the flour and a really good pinch of sea salt. Mix up as best you can, then knead vigorously on a flour-dusted surface to give you a smooth, elastic dough. Rub lightly with oil, place in the bowl, cover with a clean damp tea towel and prove for 1 hour in a warm place, or until doubled in size. Meanwhile, peel and finely slice the onion and place in a pan with the butter, bay leaves and a splash of water. Fry on a medium heat for 10 minutes, stirring regularly, then stir in 50g of flour, followed by the mustard, and slowly add the milk to give you a loose white sauce. Roughly break up the cauliflower, discarding just the tatty outer leaves, and finely slice the stalk. Add to the pan with any remaining leaves. Simmer gently for 30 minutes, stirring occasionally, then turn off the heat, grate in the cheese, season to perfection and allow to cool a little. Preheat the oven to full whack (240ºC/475ºF/gas 9). Lightly oil a 30cm non-stick ovenproof frying pan or a 25cm x 35cm baking tray, then press out the dough to fill the space. Spoon over the cauliflower mixture, leaving a 2cm border around the edge, then leave to prove again until doubled in size. Bake at the bottom of the oven for 25 minutes, or until golden, crisp and melty. Delicious served with a bowl of lemony dressed seasonal salad leaves.