Veggie shepherd's pie with sweet potato mash

INGREDIENTS

• 1 tbsp olive oil
• 1 large onion , halved and sliced
• 2 large carrots (500g/1lb 2oz in total), cut into sugar-cube size pieces
• 2 tbsp thyme chopped
• 200ml red wine
• 400g can chopped tomatoes
• 2 vegetable stock cubes
• 410g can green lentils
• 950g sweet potatoes , peeled and cut into chunks
• 25g butter
• 85g vegetarian mature cheddar , grated