If making in a blender, place sour cream, loosely packed cilantro leaves, garlic clove, lime zest and juice, and a pinch of Kosher salt in the bowl of a blender, then process until very smooth.
In a large bowl, combine all-purpose flour, rice flour, onion powder, Kosher salt, baking powder, Old Bay, and sweet paprika, then whisk well. Pour in ice cold beer, then whisk until mostly smooth.
Pat the cod portions dry with paper towels, then season on both sides with Kosher salt and black pepper.
Coat each piece of cod fully in the beer batter, using either your hands or forks to help dip and lift the fish out of the batter. Cook in batches, a few pieces at a time.
Serve immediately with tartar sauce and/or malt vinegar, lemon wedges, and fries or chips!