The ultimate makeover: Shepherd's pie

INGREDIENTS

• 1 tbsp rapeseed oil
• 1 onion , chopped
• 3-4 thyme sprigs
• 2 carrots , diced (total weight 300g/11oz)
• 250g lean minced lamb (we used 10% fat)
• 1 tbsp plain flour
• 1 tsp vegetable bouillon powder , made up to 350ml/12fl oz stock with boiling water
• 227g can chopped tomatoes
• 1 tbsp tomato purée
• 400g can green lentils with no added salt, drained
• 1 tsp Worcestershire sauce
• 650g Maris Piper or King Edward potatoes , roughly chopped
• 250g sweet potatoes , roughly chopped
• 2 tbsp half-fat crème fraîche
• 1 tbsp semi-skimmed milk