Preheat the oven to 220ºC, fan 200ºC, gas 6. Heat 1 tablespoon of the oil in a large pan and cook the onion for 5 minutes, until soft, adding in the garlic (if using) for the last minute.
Stir in the beef mince and carrots and cook for a further 5 minutes. Pour in the chopped plum tomatoes, mixed herbs and stock, bring to a boil and then simmer for 25 minutes. Meanwhile, in a medium saucepan, par-boil the sliced potatoes for 10 minutes.
Spoon the mince mixture into a 1 litre baking dish, top with the sliced potatoes, brush with the remaining oil and then cook on the top shelf of the oven for 35 minutes, until the potatoes are golden.
Meanwhile, boil or microwave the peas according to the pack instructions. Drain and serve with the cottage pie.
Chef's tip: for vegetarians, try swapping the beef mince for 454g meatfree mince.