Using a blender or food processor, process 1/4 of the sour cream with the cilantro, jalapeño pepper, and lime juice until smooth. There will still be small pieces of cilantro seen.
Pour the sour cream from the blender into a bowl, then add the other 1/4 cup of sour cream. Blend until well combined.
Add salt and pepper, to taste.
Use immediately, or refrigerate in an airtight container for up to a week. Note, the sauce will get spicier as it ages.
Feel free to experiment with more cilantro and/or more jalapeño pepper to best fit your need/taste preference. Doubling each in the recipe will create a more flavor intensive sauce, more heat, and certainly a much greener sauce.