Step 1

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.

Step 2

Heat oven to 200C/180C fan/gas 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.

Step 3

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.

Step 4

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.