Preheat the oven to 250°C and place a pizza stone or heavy baking tray on the middle shelf to heat for at least 30 minutes.
In a medium frying pan over a medium heat, cook the Italian sausage, breaking it up with a spoon, until browned and cooked through. Drain off any excess fat and set aside.
Thinly slice the green pepper, red onion and mushrooms.
Lightly dust a pizza peel or a large, flat baking tray with semolina or fine polenta.
Gently stretch or roll out the pizza dough into a circle about 30cm across, then transfer to the prepared peel or tray.
Brush the outer edge of the dough with olive oil to help the crust brown.
Spread the pizza sauce evenly over the base, leaving a 2.5cm border for the crust.
Sprinkle half the shredded mozzarella over the sauce, then top with the cooked sausage, pepperoni slices, green pepper, red onion and mushrooms. Finish with the remaining mozzarella, dried oregano and a pinch of red pepper flakes.
Carefully slide the pizza onto the hot stone or tray and bake for 10–15 minutes, or until the crust is golden and the cheese is bubbling and lightly browned.
Allow the pizza to rest for a few minutes before slicing and serving.