Sweat the fennel, leek, and shallot in the butter until soft (5mins).
Add the sliced mushrooms and cook for another 3 minutes.
Add the flour and cook out the roux for 2-3 minutes.
Slowly add the milk and cream, simmer for 2 minutes.
Add the fish, peas, dill, and lemon juice.
Bring to the boil and top with mashed potatoes.
Grill under hot grill until potatoes are nicely colored 3-5 minutes.
