Slow cooker shepherds pie

Name: Slow cooker shepherd's pie
Ingredients: 1 tbsp olive oil; 1 onion, finely chopped; 3-4 thyme sprigs; 2 carrots, finely diced; 250g lean (10%) mince lamb or beef; 1 tbsp plain flour; 1 tbsp tomato purée; 400g can lentils, or white beans; 1 tsp Worcestershire sauce; 650g potatoes, peeled and cut into chunks; 250g sweet potatoes, peeled and cut into chunks; 2 tbsp half-fat crème fraîche