Place scallions on a quarter-sheet pan fitted with a wire rack. Bake until charred and dehydrated, 20 to 30 minutes. (You can also grill the scallions, turning every 2 to 3 minutes, over medium-high heat until charred, 10 to 12 minutes.) Let cool completely.
Place scallions in a mini food processor and pulse until finely chopped. Add sour cream, adobo, agave and salt and pulse until blended. Season to taste with additional salt.
If you don't have a mini food processor, double the recipe and use a standard processor.