Preheat the oven to 180°C.
Toss potatoes in oil and spread over a baking tray. Roast potatoes in the oven for 20 minutes until lightly golden. Keep oven on for the pie.
In a large saucepan, heat oil on medium. Sauté onion and garlic for 3-4 mins until onion is tender. Add mushrooms and cook until browned.
Add lentils, potato, thyme, gravy and break up stock cube. Mix well and add S+P to taste. Transfer to a casserole dish.
Top dish with a sheet of puff pastry and bake in the oven for 30 mins or until pastry is golden.
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