Heat the oil in a large ovenproof frying pan or casserole dish over a medium heat until hot. Add the mince, onion, and carrot. Cook for 4-5 minutes until brown, breaking up the meat with the back of a spoon.
Add the garlic and herbs, stir well to combine. Cook for a further 1 to 2 minutes. Add the tomatoes, Worcestershire sauce, and stock. Stir to combine and bring to a gentle simmer. Add the lentils and stir through.
Simmer for 20 minutes until the sauce has thickened and the lentils are soft. Season, if required.
Meanwhile, cook the sweet potatoes in a pan of boiling water for 12 to 15 minutes until completely tender. Drain well and mash until smooth. Stir through half the grated cheese. Season, if required. Preheat the oven to 200°C, 180°C Fan, Gas Mark
5. Spoon the sweet potato mash evenly over the top. Scatter over the remaining cheese.
Bake in the oven for 15 to 20 minutes until the topping is golden, and the filling is bubbling at the edges.
Serve the pie straight from the dish with seasonal vegetables.