Slice the silverbeet, and separate the stalks from the leaves. Blanch the stalks in boiling water until tender. Briefly blanch the leaves. Drain and cool both stalks and leaves. Chop together, then squeeze firmly to extract any excess water. You should have a nice pile of soft silverbeet.
Heat the oil in a large frying pan and lightly fry the garlic and chilli. Add the silverbeet and toss in the oil. Season with salt and pepper to taste. Remove from the heat and cool.
Preheat the oven to 180°C. Roll out the pizza dough to a sheet of around 4 mm thickness. Arrange the prosciutto across the bottom half of the pastry, if using, then top with the silverbeet and cheese. Bring the other half of the pastry up and over the filling. Brush the edges with a little egg and press to seal. Trim away any excess pastry. Pierce the surface with a fork and brush with a little more beaten egg.
Transfer to a baking sheet and bake in the oven for 10 minutes until golden brown.