Step 1

FOR THE PICKLED RED ONION: Place onion in medium bowl. Bring vinegar, sugar, and salt to simmer in small saucepan over medium-high heat, stirring occasionally, until sugar dissolves. Pour vinegar mixture over onion, pressing onion below surface of liquid. Let onion cool completely, about 30 minutes, stirring occasionally. (Pickled onion can be refrigerated for up to 1 week.)

Step 2

FOR THE ZARANDEADO SAUCE: Process all ingredients in blender until smooth, about 20 seconds. Reserve 6 tablespoons sauce for cooking cod. Sauce can be prepared and refrigerated up to 2 days in advance.

Step 3

FOR THE FISH: Adjust oven rack to middle position and heat oven to 200 degrees. Set wire rack in rimmed baking sheet. Sprinkle cod with salt and pepper. Place flour in shallow dish. Dredge cod in flour, shaking to remove excess, and transfer to large plate.

Step 4

Heat 1 tablespoon oil in 12-inch nonstick skillet over medium-high heat until just smoking. Add 1 tablespoon butter and cook until foaming subsides. Add 6 pieces cod to skillet and cook, without moving them, for 2 minutes.

Step 5

Drizzle cod with 3 tablespoons reserved sauce and then, using fish spatula, gently flip pieces. Shake skillet gently to distribute sauce and let cod continue to cook until just cooked through, 1 to 2 minutes longer. Remove skillet from heat and, using spatula, transfer cod to prepared wire rack.

Step 6

Transfer cod to oven to keep warm. Wipe out skillet with paper towels and repeat with remaining 1 tablespoon oil, 1 tablespoon butter, 6 pieces cod, and 3 tablespoons reserved sauce.

Step 7

FOR THE TACOS: Combine red and green cabbage in bowl. Build tacos by placing 1 piece of cod in center of each tortilla, followed by cabbage, tomatoes, cilantro, sauce, pickled onions, and avocado. Serve immediately.