1. Roll out the puff pastry on a quiche dish, prick the bottom of the pastry, then cover the bottom with mustard.
2. Crumble the tuna and spread it over the bottom of the pastry.
3. Wash and dry the tomatoes, then cut them into medium slices and arrange them on top of the tuna.
4. Beat the eggs in an omelette, add the heavy cream and milk.
5. Season with salt and pepper.
6. Pour this mixture over the pastry, tuna, and tomatoes.
7. Place in the oven (preheated to 200°C/400°F), and bake for about 40 minutes (until the tart is golden without being too browned!).