To make the sourdough follow the instructions for the sourdough bread. (Link in the post and in the ingredients. It needs to be started a day ahead and fermented overnight.) The following morning take the dough out of the proofing basket and divide into 4 parts, roll into balls and put them in a well-oiled pan, cover with a kitchen towel and let them sit at room temperature for 3-4 hours.
30 minutes before you are ready to bake your pizzas preheat the oven to as high as it could go with a pizza stone inside if using. Prepare the toppings.
Take the sausage out of its casing and fry in a pan, make sure to break it up with a spatula. Season with chilli flakes, fennel seeds, basil and oregano (for Cumberland sausages only), cook until no longer pink inside, remove to a bowl and set aside.
Cook the tenderstem broccoli until bright green, no longer than 5 minutes after the water boils. Shred the mozzarella balls with your hands and grate the smoked Gouda.
Make the pizza sauce by combining the tomato passata with the olive oil, basil, oregano and salt in a medium bowl.
To shape a pizza take one sourdough ball, place it on a lightly floured sheet of parchment paper and flatten it with your fingertips into a disk, then start gently pulling with your hands from opposite side to form a thin crust, that is slightly thicker around the edges. Don't worry if it's not perfectly round!
Spread ¼ of the pizza sauce on the pizza dough, then top with cooked spicy sausage, broccoli and torn mozzarella, sprinkle with grated smoked Gouda or Cheddar. Pick up the pizza by the edges of parchment paper and transfer to the pizza stone in the preheated oven. (If you have a round cake lifter, it works very well to pick up the pizza and transfer it to the oven. You can also use a rimless baking sheet as well.) Bake for 10-12 minutes or until the crust is puffed up around the edges and the cheese is melted and slightly golden.
Repeat with the remaining 3 pizzas.