Add shrimp to boiling water, and cook until opaque, about 1 minute. Transfer shrimp to ice water using a slotted spoon, and let stand until chilled, about 5 minutes.

Add the skewers and turn to coat the shrimp. Set aside. In a medium bowl, combine the onion with the remaining 1/4 cup oil and 1/4 teaspoon salt, the parsley, sage, vinegar, and pepper.