To make the dough, place the flour in a large mixing bowl. Add salt, sugar, baking powder and yeast and stir to mix well. Add the olive oil and a little water and start to knead, adding more water until the dough clumps into a soft ball. Move it to a floured surface and continue kneading until you have a smooth dough, about 10 minutes. Cover with a tea towel and let rise 2 hours.
Meanwhile, make the sauce. Mix the diced tomato with the diced onion, oregano, olive oil and salt and pepper.
Preheat the oven to 160 degrees C.
Roll out the dough on a floured surface. Spread the sauce over half the dough leaving a border near the edge. Add the filling over the tomato sauce: onion, eggplant, zucchini, cheese, cream cheese, olives and basil.
Fold over the other half of the dough and seal the edges with water to make a pastie shape. Brush the surface of the calzone with a little water.