Knead the flour, 1/2 tsp salt, the butter, cut into small pieces and the egg quickly into a smooth dough (if necessary add 1-2 tbsp cold water). Shape into a ball and wrap in cling film. Chill in the fridge for 1 hour.
Roll out the pastry on a floured surface to a round, slightly larger than the baking tin.
Lightly fold the dough.
Place the pastry in the greased tin to line the base and sides.
Preheat the oven to 180°C (160°C fan, gas 4).
Wash the leeks, cut into rings and blanch in boiling salted water for 2-3 minutes. Rinse in ice cold water, then drain well.
Scatter over the pastry base. Whisk together the eggs and cream, season with salt, pepper and nutmeg and pour over the leeks.
Wash and halve the cherry tomatoes, then place them on the tart, cut side up. Bake in the oven for 30 minutes. Sprinkle with basil and serve in slices.