Cottage pie with vegetable mash topping

2 tbsp oil
1 med onion,chopped finely
1 clove garlic,chopped finely
1/2 x 700g pack lean beef steak mince
1 tbsp tomato puree
1 tsp dried mixed herbs
200ml beef stock
400g sweet potatoes,peeled and chopped
300g potatoes,peeled and chopped
300g carrots,peeled and chopped
30g butter
380g frozen garden peas

Heat oil in a pan and fry the onions,until soft adding the garlic at the last min.Turn up the heat add the mince and cook for 5 min.Stir in tomato puree and mixed herbs,then pour in stock.Bring to boil,then simmer for 15 min,until thickened.Preheat the oven to 200c/fan 180c/gas 6.Cook the sweet potatoe,potatoe and carrots in boiling water,drain and mash with the butter.Spoon the mince into a baking dish and top with the mash.Bake for 20 min until golden.Serve with the garden peas.