Preheat the oven to 200°C.
Bake the potatoes for 75 minutes until tender.
Scoop out the potato flesh and pass it through a fine sieve into a bowl.
Beat the potato into a fluffy purée and mix in 25g butter and 3 egg yolks.
Season with salt and pepper, pipe pyramids onto a tray, and bake for 10 minutes.
Simmer 1 litre of fish stock in a large sauté pan.
Poach the monkfish chunks in the stock for 5 minutes and then set aside.
Melt 25g butter in a saucepan and stir in the flour to form a paste.
Whisk the poaching stock gradually into the flour mixture.
Sauté onions and mushrooms in a frying pan until lightly coloured and add to the sauce.
Add the monkfish back to the sauce and return to a simmer.
Whisk 4 egg yolks and 250ml cream together and gradually whisk into the hot sauce.
Remove from heat, whisk for 2 minutes, and stir in the tarragon.
Serve the blanquette with a few duchess potatoes and extra tarragon.