Sift the flour into a large bowl, rub the butter into the flour until the mixture looks like breadcrumbs. Add the egg and cold water until the mixture resembles a firm dough. Roll into a ball, wrap in cling film and allow to chill in the fridge for approximately 30 minutes.
Grease a 22cm pie dish with butter. Roll the dough out on a floured surface and line the base and sides of the pie dish. Chill for a further 20 minutes in the fridge. Meanwhile, preheat the oven to 190°C/ gas 5. Cover with baking beans and bake blind for 20 minutes in the oven.
Remove the baking beans and return to the oven for 5-10 minutes until the pastry is golden brown.
Melt butter in a pan over a medium heat, add the leeks and fry until softened, talking care not to brown them. Set aside to cool. Spread the leeks over the base of the pastry and add the onions, peppers, mushrooms and bacon.
Beat the eggs and cream in a jug with mixed herbs and thyme. Add salt and pepper to taste and pour over the pastry. Place the tomato slices on top and finish by sprinkling the grated cheddar.
Transfer to the oven to bake for 30-40 minutes until the top is golden brown and the egg mixture has set.
Serve hot with a salad.