Preheat oven to 200°C and roast Portobello mushrooms for 20 minutes until tender.
Boil chicken stock and cook button mushrooms with the bay leaf for 8 minutes.
Boil potatoes for 7 minutes until slightly tender, then drain and slice thinly.
Sauté onion and leek in olive oil until softened.
Blend cooked button mushrooms with stock and half the dried mushroom soaking liquor.
Stir chicken into the leeks, cook for two minutes, and then stir in the flour.
Mix the blended stock, soaked mushrooms, sliced Portobellos, and tarragon into the pan.
Transfer the mixture to a pie dish and top with the sliced potatoes.
Bake for 40 minutes until golden and serve with steamed green beans.