While the onions are cooking, pour the stock into another pot and bring to a simmer.

Turn the heat up, add the sorrel leaves and a healthy pinch of salt to the pot with the onions and stir well. When the sorrel is mostly wilted, turn the heat back to medium-low, cover and cook 10 minutes. Stir occasionally. Mix in the flour and cook over medium heat for 3 minutes.

Whisk in the hot stock, stirring constantly. Bring this to a simmer.

To finish the soup, whisk together the egg yolks and cream. Temper the mixture by ladling a little soup into it with one hand, while you whisk the egg-cream mix with the other. Repeat this three times. (You are doing this to prevent the eggs from scrambling) Now start whisking the soup. Pour the hot egg-cream-soup mixture into the pot with the soup, whisking all the way. Add the final tablespoon of butter. Let this cook — below a simmer — for 5 minutes. Do not let it boil or the soup will break. Serve at once.

Lay a sheet of plastic wrap or parchment paper on a clean work surface. Using a rubber spatula, transfer the butter from the work bowl to the plastic wrap. Then, still using the spatula, form the butter into a rough log about 1 1/2 inches in diameter. Wrap the plastic wrap around the butter, roll to form a smooth, com­pact log, and twist the ends closed. Refrigerate for up to 5 days or freeze for up to 3 months.

Remove the salmon from the refrigerator 30 minutes before cooking to bring it to room temperature.

While the water is heating, rub each fillet all over with 1 teaspoon of the olive oil, then lightly season each fillet on both sides with salt and pepper. Place each fillet, skin-side down, in its own lock-top plastic bag. Cut a 1/4-inch-thick slice of sorrel butter for each piece of salmon. (Reserve the remaining sorrel butter for another use; it is delicious on chicken and steamed vegetables.) Place a disk of butter on top of each fillet.

One at a time, lay each bag flat on a clean work surface and use both hands to press down firmly on the bag all around the fillet, forcing out all of the air. Seal the bag securely.

Check the water temperature in the pan. By now it should have dropped to close to the desired temperature of 118°F (48°C). If not, add a bit of cool water. Submerge each bag in the water. (If a bag floats, lift it out of the water, open it, press out the air again, and reseal it.) Carefully transfer the pan to the oven.

Set a timer and cook the salmon for 25 minutes. To check for doneness, lift out one of the bags, carefully open it, and insert an instant-read thermometer into the center of the fillet. The salmon is done when it registers between 115°F and 120°F (45°C and 49°C).

Use a spatula to lift each fillet out of its bag and place it in the center of a warmed shallow pasta bowl. Spoon the sauce from the bag over the top. Serve immediately.