In a large heavy bottom pot on medium-high heat, sweat the onions, garlic, carrots and potato in the butter for three to four minutes, until the onions start to soften.
Sprinkle the flour in the pot and cook out for four to five minutes.
Add the milk whisking or stirring quickly until it starts to thicken.
Add the stock, bay leaves, and fresh thyme.
Bring to a boil then turn down to a very gentle simmer. Simmer for ten minutes or until the potatoes are tender.
Stir in the the cod and cook out for another seven to eight minutes.
Zest and squeeze in the lemon, stir in the parsley.