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Heat 3 tbsp oil in a frying pan, add the onion and gently cook until softened, about 10 mins. Add the thyme, vinegar, salt and pepper and cook for 5 mins more, stirring occasionally to stop them burning. Set aside to cool.
Heat oven to 200C/180C fan/gas 6. Roll out the pastry, trim to a 28cm round, then put on a non-stick baking sheet (or one lined with baking paper). Fork the pastry all over.
Spread the onion over the pastry to within 3cm of the edge. Tear the prosciutto into strips and arrange over the onion in a rippled effect. Arrange the mushrooms on top and brush them with the reserved oil. Slice or crumble the cheese and arrange over the tart. Season with salt and pepper.
Push the side of the tart up with your fingers to form an edge. Bake for 30-35 mins until the pastry edges have puffed up and the mushrooms are tender. Cool for a few minutes, then scatter with rocket just before serving.