Heat the butter in a large frying pan. Saute the leek and carrots for 5 minutes. Add peppers, mushrooms and peas. Saute for 5 minutes over medium heat.
Lower the heat and stir in flour and coriander. Add milk and cream gradually; stirring until smooth. Increase the heat, bring to the boil until the sauce thickens. Add dill, salt and pepper, stir and let cool.
Preheat oven to 180 C / Gas 4. Cut each puff pastry sheet in half to make two triangles.
Fill with cooled mixture and fold over to make a triangle squeezing the edges closed. Prick with a fork and place on greased baking tray.
Brush turnovers with beaten egg and top with a sprinkling of cheese. Bake in preheated oven for 30 minutes.
To freeze, simply place onto a tray and freeze until frozen. Place in a bag or sealable container for storage. To cook from frozen, bake for 50 minutes, but check after 40.