Combine the water, yeast, sugar, pinch of salt and 11/2 tablespoons of the oil in a large bowl. Add the flour and mix well to form a soft dough. Knead on a floured surface for 8 minutes until elastic. Cover and set aside for 15 minutes.
Meanwhile, heat remaining oil in a large frying pan over high heat. Add onions and mushrooms and cook for 8-10 minutes until liquid has evaporated. Transfer to a bowl, cool for 5 minutes. Add silverbeet and ricotta, toss to combine.
Preheat flat barbecue plate on high until hot. Cut the dough into four and roll each piece out to a rectangle 20x30cm. Spread passatta evenly over each piece of dough. Pile one quarter of the filling onto half of each piece of dough. Fold the dough over the filling and press edges together to seal.
Spray both sides of pizzas lightly with oil. Reduce barbecue heat to medium, place pizzas on the plate, close the hood and barbecue for 5-7 minutes each side until crisp. Cut into pieces and serve with lemon.