Pre-heat the oven to 200°C/400F fan and grease a medium pie dish (about 20cm).
Heat the oil in a large saucepan and add the leeks, mushrooms, garlic, thyme, oregano and salt and pepper. Stir over a medium heat for 5 minutes until just starting to soften.
Add your lentils, sweetcorn and flour. Stir until everything is coated the flour and evenly distributed (approx 3-5 minutes until no specks of flour are visible).
Stir in the milk continually over a medium heat for 7-8 minutes until the sauce has become thick. Stir in the cream and grated cheese.
Tip the sauce and vegetables into the pie dish. Brush the beaten egg around the edges of the dish and then lay the puff pastry over the top of the dish to seal. Trim to fit and then using a fork and crimp around the outside. Poke a couple of holes through the middle to allow the steam to escape.
Brush the top with the beaten egg. Bake for 20 minutes until the pastry is golden and puffed up.
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Calories: 878kcal | Carbohydrates: 87g | Protein: 35g | Fat: 46g | Saturated Fat: 18g | Polyunsaturated Fat: 5g | Monounsaturated Fat: 19g | Trans Fat: 0.01g | Cholesterol: 102mg | Sodium: 487mg | Potassium: 1390mg | Fiber: 13g | Sugar: 16g | Vitamin A: 2364IU | Vitamin C: 19mg | Calcium: 486mg | Iron: 9mg