Put sliced mushrooms on a baking tray. Mix together the mustard and olive oil and drizzle over the mushrooms. Roast mushrooms until soft and all moisture has evaporated (about 15 mins)
Rinse spinach and blanch until just losing its shape, then drain and wring out all moisture. Chop spinach finely. Mix together ricotta, spinach, chives, nutmeg and beaten eggs. Season to taste.
Roll out pastry according to pack instructions and cut into four 18cm (6″) circles. Use the trimmings to make raised borders to each tart. Place on greased baking sheets and prick the bases with a fork.
Fill each tart with the ricotta mixture and then spoon mushrooms on top. Bake for 20 minutes until golden.