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.
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.
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This recipe is simplified from BBC Good Food. Check them out for more healthy Gluten-Free recipes.