Wash chard thoroughly then remove the leaves, keeping stems intact. Cut leaves into 1-inch-wide strips and the stems into approximately 4-inch-long pieces
Season the stems with salt and pepper and place them into a vacuum seal bag with 1 tablespoon of butter and vacuum seal. (If no vacuum seal machine is available, simply use a zip-lock bag, expelling as much air as possible, then close.)
Blanch chard leaves in lightly salted water for 1 minute, then shock them in an ice bath to retain the color.
Clean clams by soaking them first in cold salted water for 10 minutes then scrubbing them to remove as much sand as possible. Discard any clams with a broken shell or ones that do not close when lightly tapped.
In a medium-sized pot, combine white wine, garlic, shallots and bring to a boil. Once liquid boils, add clams, cover with a lid and steam them for 5 minutes or until all have opened. Discard any clams that do not open. Remove clams and set aside.
Strain clam broth into a small pot and reduce by half. Add cream and sorrel and reduce to desired thickness. Once cooked, add into a blender and pulse until smooth. Season with salt, pepper and lemon juice if needed. Keep warm.
In a medium pot, heat extra-virgin olive oil to about 135° F. Do not exceed that temperature for best result. Season black cod with salt and pepper and submerge in the extra-virgin olive oil. Poach for approximately 10 to 20 minutes, depending on the thickness of the fish, until done.
To cook the chard stems, bring a pot of water to a boil and drop in the chard stems—still sealed in bag—and cook for 8 to 10 minutes.
Sauté chard leaves with remaining butter and season with salt, lemon juice and pepper to taste. Adding lemon juice brightens the flavor of the chard.
Remove clams from their shell and reheat in the sauce. Remove cooked chard stems from the bag and distribute evenly on 4 plates. Top with sautéed chard leaves and poached black cod. Finish with clam sauce and some drizzle of the olive oil.