Lightly grease a 9-inch springform cake tin or pie dish. Lay the sheet of pastry over the top of the tin, and gently ease it down into the corners. Don’t worry if you end up with a few small tears - just patch them up with more pastry. Allow the pastry to rise up the side of the tin by about 2-3 inches, and cut away any excess. Prick the base of the pastry a few times with a fork, and blind bake at 190°C (Gas Mark 5 / 375°F) for around 20 minutes, until just turning golden.
While the pastry is baking, add the red lentils to a saucepan with plenty of water or vegetable stock. Boil for around 15 minutes, until tender, then drain.
Add the cooked lentils to a large mixing bowl, along with the grated carrot, frozen peas and frozen sweetcorn (you can defrost your frozen vegetables first if you prefer - I added mine frozen). Also add the dried thyme or oregano, dijon mustard, and plenty of black pepper, and mix well to combine.
Add the egg and most of the grated cheddar to the bowl (just hold back a little cheese for topping), and mix again thoroughly.
When the pastry is starting to turn golden, remove it from the oven. If it has puffed up at all, just press it back down with the back of a spoon. Add the cheesy lentil mixture to the pie base, and top with the thinly sliced Cornish new potatoes. Finish with the last of the grated cheese and a bit more black pepper, and return to the oven for a further 30-40 minutes, until firm and golden brown.