Place the tomatoes, onion, garlic and mixed herbs or oregano in a saucepan. Bring to the boil, then reduce the heat and simmer gently for 15 minutes until the onion is softened and the sauce is thickened, stirring regularly. Cool.
while the tomato sauce is cooking, boil the sweet potato in a large pan of water for about 15 minutes until very soft. Drain well, return to the pan and mash with a potato masher until smooth. Set aside to cool.
Preheat the oven to 200C/Gas Mark 6 400°F Place 375g of the sweet potato in a large bowl. Add the flour, baking powder and seasoning.
Stir in the water and oil mixture with a large spoon until the dough comes together it should be soft and spongy. Knead lightly to form into a large ball - adding a little extra flour if the mixture seems too sticky.
Divide the dough into two equal balls and roll out on a lightly floured board into two circles around 2cm thick. Lift carefully onto two lightly oiled baking sheets. Brush lightly with oil. Spread one of the pizzas with half of the tomato mixture, leaving a 2cm gap around the edge. Top with half the peppers, zucchinis, onion and olives. Shake on some oregano.
Place both pizzas in the oven, with the topped pizza above the plain base. Bake the topped pizza for around 25 minutes until well risen and lightly browned. Cook the plain base for 15 minutes until golden and cooked then remove and allow to cool.
Serve the topped pizza hot garnished with oregano and lots of fresh, raw basil leaves, accompanied by a fresh, crunchy mixed salad. (Sometimes I toss over raw spinach leaves too).
Spread the remaining tomato mixture over the plain base,top with reserved vegetables and olives, cover loosely in foil and freeze on the tray. The next day remove from tray and wrap tightly in foil. To cook, remove foil, place on a lightly oiled baking tray and bake from frozen in a preheated oven at 400F/200C/Gas 6 for 15-18 minutes until hot.