1. Mince the onion finely.
2. Slice the leek into thin rings.
3. Mince the garlic as finely as you can.
4. Heat a large skillet over medium-high heat. Add the oil.
5. Add the onion and leek and cook for 3-4 minutes until they start to soften.
6. Add the garlic and cook for a minute.
7. Add the ground beef and season with 1 teaspoon salt. Cook until the meat is browned (about 10 minutes).
8. Push the meat to one side of the pan. Turn up the heat to high.
9. Pour in the Worcestershire sauce and let it bubble for 30 seconds, then stir it into the meat.
10. Add the tomato ketchup and water. Reduce heat to medium and simmer until most of the liquid has evaporated.
11. Set aside to cool while you make the mashed potatoes.
12. Peel the potatoes and cut them into 2-2.5cm cubes.
13. Boil the potatoes in salted water for 20 minutes, or until tender.
14. Mash the potatoes or put them through a potato ricer.
15. Add the butter and ¾ cup of the cheddar cheese. Mix well.
16. Divide the meat mixture between 2 (15x10cm) gratin dishes.
17. Top with the mashed potatoes and use a piping bag or simply spread them over the meat.
18. Sprinkle the remaining cheese and breadcrumbs on top.
19. Bake at 400°F for 20 minutes, or until golden brown.
20. Let cool for 5 minutes before serving. Garnish with chopped spring onions.