Cook the potatoes in a pan of boiling water for 12 mins or until tender. Drain and return to the pan. Season, then lightly crush with a potato masher or fork. Stir in half the parsley.
Meanwhile, heat 2 tablespoon of the oil in a large, deep frying pan over a medium heat, then add the onion and cook, stirring often, for 2-3 mins. Increase the heat to medium-high, add the mushrooms and fry for 7-8 mins, stirring occasionally, until softened.
Add the mushrooms and fry for 7 mins, stirring occasionally, until softened and they've released their juices.
Stir in the water, garlic, dried crushed chillies, lentils, chopped tomatoes and tomato purée, then reduce the heat and simmer for 8-10 mins or until the sauce has thickened, then add the remaining parsley and season with the salt and pepper. Preheat the grill to high.
Pour the lentil mixture into a large baking dish, then spoon the crushed potatoes over the top. Drizzle with the remaining olive oil and grill for 8-10 mins or until the potatoes are golden and begin to turn crispy.
Freezer-friendly. We would recommend cooling your leftovers as quickly as possible, ideally within two hours before freezing and then eating within 24 hours of fully defrosting.Adapted with permission from Tesco