Preheat a grill to high. Soak six 10-inch wooden skewers in water for at least 30 minutes.
Meanwhile, make the pesto: Pulse the walnuts, lemon zest and juice, olive oil and a pinch of salt in a food processor until the walnuts are in small pieces. Pulse in the scallions, basil and parsley until fairly smooth. With the machine on, drizzle in 2 to 4 tablespoons cold water to loosen. Transfer to a bowl and season with salt.
Make the shrimp: Thread the shrimp through the tail and top end onto the prepared skewers so they lie flat. Brush with the vegetable oil and season on both sides with salt and the red pepper flakes. Grill the shrimp on the hottest part of the grill until marked, 3 to 4 minutes; flip and grill until cooked through, 2 to 3 more minutes. Remove from the grill and drizzle with the red wine vinegar. Remove the shrimp from the skewers, if desired, and serve over mixed greens. Drizzle with some of the pesto and serve the remaining pesto on the side.
Photograph by David Malosh