Put the dried porcini into a large bowl and cover with boiling water. Set aside to soak for 30 minutes.
Preheat the oven to 200C / 180C Fan / Gas Mark 6. Cook the flatbreads according to the pack instructions.
Heat the oil in a large frying pan and cook the portobello and button mushrooms for 6-8 minutes until softened and beginning to brown.
Drain the soaked porcini, reserving 1 tablespoon of the liquid. Chop and add to the frying pan. Cook for 2 mins.
Stir the reserved porcini liquid and the creme fraiche into the mushrooms. Add the leaves from 1 spring of thyme, season with black pepper then spoon onto the flatbreads.
Cut each flatbread into 3 pieces, then sprinkle with the remaining thyme before serving.