1.Preheat oven to 220°C (200°C fan-forced). Bring a saucepan of salted water to the boil. Line trays with baking paper.
2.Melt butter in frying pan over medium-high heat. Add leek, mushrooms, thyme and garlic. Cook 15 minutes, stirring occasionally. Stir through cream, salt and pepper. Cook further 2-3 minutes. Cool 10 minutes, then remove thyme sprigs.
3.Meanwhile, while mushroom mix is cooking, place potato slices in boiling water. Cook 6-8 minutes, until just tender. Drain. Cool.
4.Cut 4 x 15cm discs and 4 x 12cm discs from puff pastry. Place 2 smaller discs on each lined baking tray.
5.Lay potato slices evenly on pastry bottoms, leaving a 1.5cm border around edge. Brush pastry border with egg. Divide mushroom mix evenly among 4 pithiviers. Top with larger pastry discs; press edges of each pithivier firmly together, to seal. Brush with egg.
6.Bake 25-30 minutes, until pastry is puffed and golden. Swap trays around on racks after 15 minutes. Serve with salad.