Preheat your oven to 220°C/200°C fan/gas mark 7. Bring a large saucepan of water to the boil with 1/2 tsp salt for the potatoes.
Chop the potatoes into 2cm chunks (peel first if you prefer). Thinly slice the mushrooms. Roughly chop the chives (use scissors if easier). Peel and grate the garlic (or use a garlic press).
Drain and rinse the lentils in a sieve. Grate the Cheddar.

Halve any large cauliflower florets and pop them onto a large baking tray. Drizzle with oil, sprinkle over the ground cumin and season with salt and pepper. Toss to coat, then spread out in a single layer.
When the oven is hot, roast on the top shelf until golden brown and tender, 15-20 mins. Turn halfway through.

While the cauliflower roasts, add the potatoes to the boiling water and cook until you can easily slip a knife through, 15-20 mins.
Meanwhile, heat a drizzle of oil in a large frying pan on medium-high heat.
Once hot, add the mushrooms and season with salt and pepper. Cook until browned, 5-6 mins.
Add the garlic and harissa paste (add less if you'd prefer things milder) and cook for 1 min more.

Pour the chopped tomatoes, lentils, veg stock paste and water for the sauce (see ingredients for amount) into the mushrooms, stirring to combine.
Bring to the boil, then lower the heat to medium and simmer until thickened, 8-10 mins.
Once the cauliflower has roasted, stir it through the sauce, then remove from the heat. Add a splash of water if it's a little thick.

Increase your oven temperature to 240°C/220°C fan/gas mark 9.
Once the potatoes are cooked, drain in a colander and return to the pan, off the heat.
Add a knob of butter and a splash of milk (if you have any) and mash until smooth. Season with salt and pepper, then stir through half the chives.

Taste and season your pie filling if needed, then transfer it to an appropriately-sized ovenproof dish. Top with an even layer of mash and sprinkle over the cheese.
Bake on the top shelf of your oven until golden and bubbling, 10-15 mins.
Allow to stand for 2 mins before serving and sprinkle over the remaining chives to finish. Enjoy!