Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill and boil your kettle. Tip the dried porcini mushrooms into a heatproof jug and cover with boiling water. Leave to soak for 15 mins, then drain and rinse with cold water. Pop the porcini back into the jug and cover with 125ml boiling water.

Peel and finely slice the onion. Warm 1 tbsp olive oil in an ovenproof frying pan on a medium-high heat (see our tip if your pan isn't ovenproof). Add the onion and lower the heat. Cook for 5 mins, stirring now and then, till soft and glossy.

Peel and grate the garlic. Tear the chestnut mushrooms into small pieces and add to the onion. Season and cook on a medium-high heat for 4 mins, till the mushrooms look juicy. Add the garlic to the pan and cook for 1 min.

Add the porcini and their stock to the pan. Cook and stir for 1-2 mins. Take off the heat. Stir the crème fraîche and 2 tsp mustard through the mushrooms. Season with salt and pepper.

Finely chop the flat leaf parsley, leaves and stalks. Peel and coarsely grate the potatoes into a large bowl. Add half the parsley and some salt and pepper. Pour in 1 tbsp olive oil and turn to mix. Scatter the mixture over the mushrooms. Slide the pie into the oven on a high shelf and bake for 30 mins, or till the top is golden and crisp.

While the pie bakes, zest and juice the lemon. Tip the zest into a large bowl. Whisk in half the juice, 2 tbsp olive oil and a pinch of salt. Peel the beetroot. Thinly slice it and fold it through the dressing. Set to one side.

When the pie is ready, divide the lamb's lettuce between 2 warm plates and top it with the beetroot, drizzling over the dressing. Serve with the pie, garnished with the rest of the parsley.