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Heat the oven to 220C/200 fan/gas 7 and put two baking sheets inside.
Make the sauce: heat the oil in a small saucepan and cook the onion with a generous pinch of salt for 10 mins over a low heat until softened. Add the chopped tomatoes, purée and sugar and bring to a gentle simmer. Cook, uncovered, for 25 - 30 mins or until reduced and thick, stirring regularly. Blitz the sauce with a hand blender until smooth. Season to taste and stir through the basil. Allow to cool a little.
Make the dough: mix the flour, sugar, baking powder, salt and xanthan gum in a large mixing bowl. Make a well in the centre and pour in 250ml warm water and the olive oil. Combine quickly with your hands, to create a thick, wet, paste-like texture, adding an extra 20ml warm water if the dough feels a little dry. Store in an airtight container or covered bowl in the fridge for up to 24 hours before using. Lightly flour two more baking sheets. Split the dough into two and flatten with your fingers into 20 - 25cm rounds on the sheets.
Finish the bases with a thin layer of the sauce and torn up mozzarella. Place the baking sheets on top of the hot baking sheets in the oven and cook for 8 -10 mins or until crisp around the edges.