How to Make It
In a small food processor, combine the sour cream with the cilantro, garlic, water and 1 teaspoon of salt and puree until smooth.
Light a grill and brush the grates with olive oil. Grill the scallions over high heat, turning occasionally, until lightly charred in spots, about 4 minutes. Transfer to a work surface and let cool. Trim off the roots and coarsely chop the scallions.
Brush the grill grates with more olive oil and season the fish with salt. Grill the fish over high heat, turning once, until charred on the outside and opaque throughout, about 4 minutes per side.
Grill the tortillas until warm and lightly charred, about 30 seconds per side. Spread some of the crema on the tortillas and top with the fish and scallions. Fold the tortilla over the filling and serve right away, passing more crema at the table.