First make the pesto. Add arugula, green onions, lemon juice, water, sour cream, mayonnaise, garlic, pepper, salt, basil, and red pepper flakes to a food processor. Pulse until smooth. Taste it and see what you think!
Pat salmon filets dry and sprinkle the skin side with salt and pepper. Dust each filet with about a half teaspoon flour.
Spread each with about 1 teaspoon of the arugula pesto.
Heat olive oil in a large skillet over high heat.
When the oil is very hot, place 2-3 salmon filets skin (and pesto) side down in the hot skillet. (Don't crowd the pan, you don't want them to steam. Do it in batches if you don't have room. I used a 12-inch skillet and only did 3 at a time.)
Cook over high heat for 1 1/2 to 2 minutes, or until there is a nice sear on the bottom. Sprinkle the top with salt and pepper, flour, and another teaspoon of pesto, then flip.
Cook for 30 seconds to 1 minute and then remove from heat. Let sit in the pan for another couple minutes.
Serve on a bed of arugula, with a fresh squeeze of lemon and a few spoonfuls of the pesto.
This is really great with a green salad, crusty bread, roasted potatoes, or even some simple pasta.