Beat whole eggs with sour cream in a large bowl and season with salt and pepper.
Add scallions, salmon, thyme and mix thoroughly. In a medium bowl, beat egg whites just until soft peaks form.
Fold into the egg mixture until well combined. Melt 2 tablespoons of butter in a 10" ovenproof skillet with a lid over medium heat.
Pour egg mixture into pan, cover and reduce heat to moderately low. Cook until eggs are set, 20 – 25 minutes, until frittata is puffed and the center springs back when touched.
Pistou Sauce: While the frittata is baking, combine the sorrel, shallot, salt, and pepper to taste in the bowl of a food processor. Pulse until finely chopped, add olive oil and the remaining tablespoon of butter. Process until smooth, taste for seasoning, then transfer to a bowl and set aside.
Place under the broiler if desired for golden color.
Drizzle the frittata with pistou.
Slice the frittata and serve with more pistou.
A meal that can be served at any time of the day.