1. Prepare an ice bath; set aside. Bring a medium saucepan of water to a boil, and add 2 1/2 teaspoons salt and arugula. As soon as the water returns to a boil, remove from heat. Using a slotted spoon, transfer arugula to the ice bath to stop cooking. Working in batches, remove arugula from ice bath and place on paper towels to drain, squeezing out as much excess water as possible.

2. Transfer arugula to the jar of a blender. Add oil, remaining 1/4 teaspoon salt, and pepper; puree until mixture is smooth and resembles a thick pesto with a small amount of unincorporated oil on the surface. Before using, stir gently to combine.