Heat your oven to 200°C/Fan 180°C/Gas 6. Pop your porcini into a small heatproof bowl and cover with 125 ml boiling water. Leave to soak for 10 mins.
Peel and finely slice your onion. Heat 1 tbsp olive oil in an ovenproof frying pan over a medium-high heat. Add the onion and lower the heat. Cook for 5 mins or till tender and glossy.
Peel and grate your garlic. Finely slice your mushrooms. Add them to the onion. Season. Cook over medium-high heat for around 5 mins or till the moisture has cooked out of the mushrooms. Add the garlic for the last 1 min.
Add the porcini and its soaking liquid to the pan. Cook and stir for 1-2 mins. Take off the heat. Stir the crème fraîche and mustard through the mushrooms. Season to taste.
Finely chop your dill. Peel and coarsely grate your potatoes into a large bowl. Add half the dill. Season. Pour in 1 tbsp oil. Scatter the mix over the mushrooms. Pop in the oven on a high shelf. Bake for 30 mins or till the potatoes are golden and crisp.
Grate the zest from your lemon. Juice it. Pop the zest in a bowl. Whisk in 2 tbsp lemon juice, 2 tbsp olive oil and a pinch of salt. Peel the beetroot. Thinly slice. Fold through the dressing. Put to one side.
When the pie is done, arrange the beetroot slices on a plate. Keep hold of the beetroot dressing. Pile the watercress in the centre of the plate. Drizzle with the remaining dressing. Serve the pie with the salad. Scatter with the rest of the dill.