To make the mince, place a large saucepan over a low heat. Add the oil and once hot, sauté the onion, carrot, celery and garlic with the herbs until softened. Add the tomato puree and plain flour. Pour over the beef stock and bring to a boil. Reduce the heat and simmer for 20 minutes on a low heat. Once cooked, stir through the peas.
Meanwhile, place the potatoes in a pan of salted water and bring to the boil, cooking for 20 minutes. Drain the potatoes and add the butter, milk and 50g of grated Dubliner White Mature. Mash until smooth.
Preheat your oven to 180C/160C fan/gas mark 5.
To assemble the cottage pie, fill a baking dish with the mince and top with mashed potato. Use a fork to rough up the top of the potato - this will allow for more colouring. Transfer to the oven to bake for 25 minutes or until golden and bubbling.