1. Clean the carrot, onions and celery and roughly chop them into chunks.
2. Place the vegetables plus 2 cloves garlic into the food processor with the blade attachment. Blitz them until they are finely chopped. This should take less than 15 seconds.
3. In a large saucepan, saute the minced beef with 2 tbsp. of oil until all the moisture evaporates and the mince starts frying. Break the mince with a wooden spoon.
4. Once the mince starts to brown, add in all the chopped vegetables from the food processor and saute the veggies until soft and slightly brown.
5. Add in the tomato paste and cook it for a couple of minutes. Pour in the water, crumble in the stock cube and mix herbs.
6. Scrape the crispy bits at the bottom of the pot. Cover with a lid and simmer for an hour. Stir occasionally.
7. Season with salt, pepper and Dijon mustard.
8. Peel the potatoes and put it through the food processor chute with the slicer blade attachment.
9. Layer the sliced potatoes and meat sauce in a wide oven proof dish. Repeat layer until you have used up all the sauce and potatoes.
10. Sprinkle grated cheese on top just before baking at 180 degrees for half an hour.
11. Let the gratin cool for 15 minutes before serving.