Sweet potato shepherd's pie

Name: Sweet potato shepherd's pie
Prep time: 30 min
Total time: 30 min
Cuisine: European, British
Ingredients: 2 tsp vegetable oil; 1 large onion, finely chopped; 250g mushrooms (any kind), diced; 2 garlic cloves, crushed; 1 x 500g pack 10% fat lamb mince; 2 medium carrots, grated coarsely; 2 celery stalks, grated and any strings discarded; 1 tbsp cornflour; 2 tbsp tomato purée; 4 sprigs thyme, leaves picked; 1 tbsp Worcestershire sauce (optional); 1 x 400g tin green lentils, rinsed and drained; about 900g sweet potatoes, peeled and chopped; 50ml coconut or soya milk (or cow’s milk, if no DF requirement)