Prep - Rinse sorrel or arugula and pat dry. Pack leaves tightly and slice crossways into very thin filaments. Place in a bowl. If using Arugula, squeeze in juice of ½ lemon and toss. Using a sharp thin knife, cut the salmon across the grain on the bias into 6 thin slices. Check for any small bones.
Make Sauce- In a small saucepan, add wine, fish stock, 1 tbsp scallion, and ¼ of the diced tomatoes. Simmer over medium heat until reduced by half, about 5 minutes. Add cream and reduce liquid to 1 cup, about 5 minutes more. Strain through a coffee filter in a wire mesh strainer or colander placed over a bowl. Press down to extract as much liquid as possible.
Put 1 tbsp butter back in pan and add the the remaining 1 tbsp scallion. Stir then add sorrel or arugula and cook until wilted. To the wilted sorrel add the strained cream mixture, remaining cup cubed tomatoes, salt and pepper.
Cook Fish - Heat the remaining butter in the large non-stick pan. Season the fish slices and add to the pan in 1 layer. Cook about 1 minute per side.
Finish Dish - When the salmon first goes into the pan, pour sorrel sauce into the center of a serving platter and smooth it around. Arrange salmon slices over the sauce as they finish cooking, overlapping slightly. Sprinkle with parsley and serve immediately.