Mix the flour, salt and yeast in a bowl. Add the oil and water, knead into a soft dough. Grease a bowl with a little olive oil, add the dough, cover and leave to rise at room temperature for approx. 1½ hrs. until doubled in size.
Drain the cashew nuts, transfer to a high-power blender with all the remaining ingredients and puree for approx. 1 min. until completely smooth. Gently heat the mixture in a small non-stick pan and bring to the boil, stirring constantly with a silicone scraper. As soon as the mixture thickens, simmer and stir vigorously for approx. 2 mins. Immediately remove the pan from the heat, leave the mixture to cool and set.
Cut the dough in half. On a lightly floured surface, roll out the dough into 2 circles (each approx. 30 cm in diameter), transfer to a baking tray lined with baking paper. Spread tomato sauce on top of the dough, top with the onions and tomatoes. Spoon on the cashew cheese, season the pizzas.
Bake for approx. 15 mins. in the lower third of an oven preheated to 200°C. Remove from the oven, garnish with basil.