Mix flour, semolina, salt and sugar thoroughly in the mixing bowl of the food processor. In a second bowl, dissolve the yeast in the water and add to the flour mixture.

Add olive oil and mix the dough for 2 minutes at a low speed until there is no loose flour in the bowl. Knead for another 8 minutes at medium speed, or until the dough is smooth and ceases sticking to the sides of the bowl. (You can also use a spatula to remove the dough from the sides if needed.)     

Cover the dough and leave to rise for 2 hours at room temperature (or, alternatively, overnight in the fridge).

Roll the sausage meat into walnut-sized balls (about 1 inch in diameter, or 2 ½ centimeters) with wet hands to make about 12 balls. Set aside. 

Warm the clarified butter in a frying pan over medium-low heat. Add the onions and fry until soft, about 10 minutes. Add kale and cook for another 10 minutes, stirring until most of the liquid has evaporated.

Season the mixture with salt, pepper and mustard and leave to cool slightly. Stir in grated cheese and eggs.

Brush the skillet with oil. Preheat the oven to 260 °C. Use an infrared thermometer to gauge the temperature of the stone inside.

On a flour-dusted surface, divide the dough into 2 equal portions. Using a rolling pin, roll each one out into 10-inch (25-centimetre) rounds.

Place the pizza dough into the cast-iron skillet and press it into the bottom and up the sides. Leave the dough to rest for 10 minutes.

Place the skillet with the dough in towards the back of the oven, letting it cook for 5 minutes. Turn it and continue cooking until the surface of the pizza is slightly and evenly browned.

Remove the skillet from the oven and top with half of your kale mixture and six mett balls. Place it back in the oven, towards the front, and continue baking for another 20 minutes, rotating every few minutes for an even cook. If the top is browning too fast, turn off the oven, move the skillet to the middle and close the door to continue baking the pizza in the residual heat.

Remove the skillet from the oven and let the pizza rest for 5 minutes. Slice while still in the pan and serve piping hot.