Preheat the oven to the highest setting it can go.
Find your biggest baking tray and an oven proof pyrex bowl and grease both. Roll out the dough until its big enough to drape over the bowl and the tray. Make sure there's a hollow of dough for the cheese to be poured into.
Coat the base of the volcano with tomato sauce and mozzarella then sprinkle over a few basil leaves.
Bake for around 20 minutes, until the cheese is bubbling and the dough has crisped up.
Meanwhile, make your sauce. Melt the butter over a low flame and add the flour. Cook out for 1 minute. Gradually add the milk while whisking until all incorporated. Pour in the hot sauce and harissa then bring up to heat. Remove from the heat and whisk in the cheese bit by bit until nice and smooth. Season if necessary!