In a bowl, mix together the dough ingredients and shape into a big ball. Leave the dough to rest for around 10 minutes. Then separate the dough into 3 individual balls. Pop into the fridge for at least 4 hours to chill. We always find it's better to make this the day before.
In a pan, gently warm the milk. In a separate pan, put in the butter to melt, and add the flour. Stir together to combine it into a paste which is more commonly known as a roux. To the roux, slowly pour in the warm milk, stirring continuously in order to ensure there is no lumps. Add the cheese, mustard and a pinch of salt into the sauce and stir until smooth. Set to one side whilst you get the other ingredients ready.
Roll out the dough to approximately 1 cm thick. Ideally a circle but any shape will be just as delicious! Spread the warm cheese sauce over the top – leaving a gap of about half an inch away from the edge of the crust.
Lightly scatter the chicken, leek and cheese and thyme and drizzle of olive oil onto the pizza.
Cook in a pre-heated pizza oven or place a tray into the oven at the highest possible temp and then slide the pizza onto the tray. This will cook very quickly - about 10 minutes at 240C.