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FILL a medium-sized and a large pan with water and salt and bring them both to a boil. Meanwhile CHOP the leeks into pieces, not too big. ADD the chopped leek to a colander and rinse them well.
COOK the tagliatelle in a large pan, use a fork to loosen up the tagliatelle nests. COOK the chopped leeks about 5 minutes in the medium-sized pan.
PLACE the colander on top of a large bowl, this way you can separate the liquid without the solids. SET the cooked leek aside and measure about 300 ml of the liquid and set that aside too.
MELT 40 gram butter in the medium-sized pan and ADD flour all at once to create a roux. Gradually ADD the leek liquid and keep on stirring with a whisk. As soon as the sauce is smooth, it will also thicken.
ADD pesto, cream and crème fraîche and if that's smooth again you can also add the cooked leek. Meanwhile HEAT butter or oil in a large skillet.
If you'd like you can COAT the fish fillets with a layer of flour first before you cook them for a few minutes on both sides. SEASON the leek-pesto with pepper and salt.
DRAIN the tagliatelle and serve with sauce and cod fillet. Enjoy!