This dish is all about the flavorful sauce that can be whirled in a blender in a matter of minutes. A combination of fresh cilantro, avocado, scallions, and lime juice, it's a terrific way to brighten up a plate of skewered shrimp. If you don't care for cilantro, make the sauce with fresh basil instead.
Make the sauce by putting the cilantro (or basil), scallion, avocado, water, lime juice, vinegar, salt, and chile flakes in a blender and run until smooth, scraping down the sides and running again as needed.
Thread 3 to 5 shrimp on a wooden skewer. Repeat with remaining shrimp. Heat the grill on high (alternatively turn on the broiler in your oven). Cook the shrimp on the grill or on a sheet pan under the broiler until pink and opaque, 2 to 3 minutes per side.