Preheat the oven to 220°C, fan 200°C, gas 7. Cut a 23cm circle from the pastry and lift onto a baking sheet. Prick the pastry all over with a fork and brush the top with beaten egg - but don't brush the very edge or it won't rise evenly.
Bake for 10 minutes. Meanwhile, mix the oil, spring onions, pesto and tomato purée together in a small bowl. Spoon the pesto mixture over the baked pastry round, spreading it to within 2cm of the edge.
Top with the tomato, courgette and mushroom slices and sprinkle with the Cheddar. Return to the oven for 10 minutes, or until the tomatoes and courgettes have softened and the cheese has melted. Cut into wedges and serve immediately.