Remove skin from potatoes. Fill a medium sized saucepan with water and add the 4 potatoes. Boil on medium heat.
Heat olive oil in large saucepan, add chopped onion and leek and simmer for 10 minutes making sure it does not brown. Stir in flour- you can see the sauce will immediately start to thicken up. Add 2 cups of the vegetable stock and then all of the chopped mushrooms. Simmer for 10 minutes. Add rest of stock if it's too thick. Add the thyme, pepper, cream and stir. Simmer for another 10 minutes. Keep checking to make sure it does not stick to the pan.
Drain potatoes, chop into edible chunks and add to saucepan. Add the lentils, stir. Simmer for 5 minutes or until desired consistency, making sure it's not too watery. Turn heat off and let it sit for 10 minutes to thicken up. Meanwhile take puff pastry out of the freezer and let it thaw.
Get out a large glass bake dish and pour the mixture in, then top with puff pastry. Optional- brush with butter (or egg). Cook in 180 degrees C oven until the puff pastry is golden brown (about 30 minutes). Serve from the baking dish.
Instead of cream, try adding a powdered gravy at the same time as you put the flour in.