Preheat the oven to 190°C (170° fan) | 375F | gas 5.
Roll out the pastry on a floured surface and use it to line a 23 cm | 9-inch round spring-form tart tin. Prick the pastry with a fork, line with clingfilm and fill with baking beans or rice. Bake for 10 minutes then remove the clingfilm and baking beans. Brush the inside with a little beaten egg and return to the oven for 8 minutes to crisp.
While the pastry is cooking, steam the cauliflower and broccoli for 5 minutes or until just tender.
Remove the pastry from the oven and lower the temperature to 150°C (130° fan) | 300F | gas 2.
Whisk the rest of the beaten egg with the double cream and mustard until smoothly combined, then stir in the ham and diced yellow tomatoes. Season generously with salt and pepper.
Arrange the cauliflower, broccoli and halved red tomatoes in the pastry case, then pour over the egg mixture.
Bake for 40 minutes or until just set in the centre. Leave to cool for 15 minutes, then remove the sides of the tin and serve.