The ultimate makeover: shepherd's pie

Name: The ultimate makeover: shepherd's pie
Prep time: 25 min
Cook time: 1 h 25 min
Total time: 1 h 50 min
Cuisine: British, European
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 lentil with no added salt, drained; 1 tsp Worcestershire sauce; 650g Maris Piper or King Edward potato, roughly chopped; 250g sweet potatoes, roughly chopped; 2 tbsp half-fat crème fraîche; 1 tbsp semi-skimmed milk