Warm your oven to 200C/Fan 180C/Gas 6. Fill a large pan with water and add a good pinch of salt. Peel and chop the sweet potatoes into small chunks and slide them into the pan. Pop the pan onto a hob and bring to the boil. Simmer the sweet potatoes for 10-15 mins, till tender.

Meanwhile, peel and finely chop the onion. Scrub and finely chop the carrot. Trim the ends off the celery and finely chop it. Peel and thinly slice the garlic cloves. Pick the rosemary leaves and roughly chop them. Tear the mushrooms into small chunks.

Melt half of the butter in a large pan (keep the rest for later in the recipe) over a medium heat. Slide in the chopped onion, carrot, celery, garlic, mushrooms and rosemary. Sprinkle with a good pinch of salt. Fry the veg for 6-8 min, stirring occasionally, till juicy and softened.

Drain the lentils and rinse them under a cold tap. Shake the lentils dry then tip them into the pan with the cooked veg. Pour in 1 tbsp worcester sauce and crumble in the veg stock cube. Pour in 100ml boiling water and stir well. Warm through for 5 mins.

Grate the cheddar. When the sweet potatoes are tender, drain them well and allow to steam dry for 2 mins. Tip the sweet potatoes back into the pan and add the reserved butter, the grated cheddar and a pinch of salt and pepper. Mash the sweet potatoes well till smooth.

Transfer the lentils and veg into a deep oven dish. Top with scoops of the cheesy sweet potato mash and smooth down with a fork. Slide the dish into the oven and bake for 20-25 mins, till bubbling and golden. Divide the pie between plates and serve.