Pre-heat the oven to Gas Mark 6, 200°C, 180°C fan.
Prepare the sauce by heating the olive oil in a saucepan over a moderate heat. Sauté the garlic for a minute, then add the passata, sugar and some seasoning. Bring to a simmer and cook for 4-5 minutes until thickened. Remove from the heat.
Place the pizza bases on a baking tray (or two, depending on the size of your oven) and spread the tomato sauce evenly between all four bases. Sprinkle the grated mozzarella on top, followed by the mushrooms, red onion and ham. Bake for 8-10 minutes or until the cheese has melted and the edges of the pizza bases are just turning golden.
Remove from the oven and garnish with a sprig of thyme before serving, if desired.
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