Combine all the ingredients (except mushrooms) in a vacuum seal bag and seal.
Cook for 30 minutes.
Heat a skillet over medium heat and add the mushrooms.
Remove the fish from the bag and set aside. Add the bag juice to the mushrooms in the skillet and cook for 2-3 minutes. (If the sauce is too thick, add some additional butter).
Spoon the mushroom sauce over the fish and serve.
**If using dried shiitakes, soak them in hot water for 30 minutes, until they are softened. Squeeze the excess water out and slice, discarding the stems.
To ensure the cod fillets cook evenly, make sure they’re all the same thickness. (The loin part of the fillet is usually thicker than the tail end).
If your fillets are thicker, you may need to increase the cooking time by about 15 minutes.
You’ll want to make sure the fillets are arranged in the sealable bag in a single layer. This will help them cook evenly.
For maximum flavor, make sure the butter is evenly distributed around the cod before the bag is sealed.
When sealing the bag, make sure the air is removed to avoid bacteria getting in the bag.
The sealed bag should also be completely submerged in the water bath. I like using something heavy like a sous vide sinker weight to prevent it from floating.
If the bag starts to float due to air getting in, just open the bag, remove the air and reseal it.
Keep the bag juice (the juice from the bag the cod is cooked in) to make the mushroom sauce. If the sauce turns out too thick, just melt in some more butter.
You can substitute dried shiitake mushrooms if you can’t find any fresh. Just soak them in hot water for 30 minutes until they’re softened. Squeeze the excess water out, slice them and discard the stems.