** If you are making your own vegetable stock [see Note 3], get this on the heat now!! Preheat your oven to 200 C [395 F]
Gather and weigh your ingredients
Add the smoked haddock to a frying pan, cover with cold water and bring to a simmer and poach the fish for 3-4 minutes, or until the fish starts to flake. Lift the fish out to a plate. ~When it is cool enough to handle, remove the skin if applicable and fake the fish into chunks and set aside. Discard the fish water and wipe the pan with a paper towel.
Add the sliced leeks to the fry pan and add a knob of butter over medium heat. Cook the leeks until starting to wilt slightly, about 5-6 minutes. Add a ladle full of the boiling stock into the pan and add your vegetables, continue cooking for another 3-4 minutes until your vegetables are soft-ish [Note 5] Drain and set aside.
Now to make your roux - add the 3 Tablespoons butter to the saucepan, over medium heat. Using your whisk, start stirring the butter and with the other hand, pour in the 3 Tablespoons of flour- whisk into a nice, creamy paste and cook for a couple of minutes, stirring constantly [ long enough to cook out the flour taste]- turn the heat down to low/low-medium ** [add the cooked garlic clove and smash it now in the pan] Add the 250 ml stock + the gratd ginger root and stir - you want this to be a creamy consistency [add/make more roux if required, but you don't want this to get too thick- err on the side of caution and keep it thinner versus thicker, if that makes sense as it will thicken up in the cooking process]
Stir through the blue cheese, creme fraiche [or soured cream] and add the cooked/tinned butter beans. Mix thoroughly, then add the cooked leeks, flaked fish, vegetables, tarragon and salt & white pepper.
Meanwhile, in a separate bowl, combine the dried breadcrumbs, parmesan or cheddar cheese and a pinch or tow of the remaining tarragon.
Pour the fish mixture into a prepared gratin dish. Sprinkle the breacrumb/cheese mixture over the top and place in the oven on the middle rack. Cook for 15-20 minutes until the sauce is bubbling slightly and the top is golden.