Season the cod loins with salt.
Put the cod in a sous vide bag together with olive oil and a few sprigs of fresh dill.
Cook for 30 minutes at 130°F.
Combine the wine, onion, dill stems, and black pepper in a saucepan.
Whisk in heavy cream and simmer to thicken a bit.
Season with salt if needed.
Fold in chopped dill.
Serve sauce over the cod.