San Diego Fish Tacos

Ingredients

• 1 red onion, halved and sliced thin
• 1 cup distilled white vinegar
• ⅓ cup sugar
• 1 teaspoon kosher salt
• ⅔ cup mayonnaise
• ⅓ cup sour cream
• 1 chipotle chile in adobo sauce
• 1 arbol chile, seeded
• 1 tablespoon Tapatío hot sauce
• 1 tablespoon beer
• 1 tablespoon yellow mustard
• 1 garlic clove, chopped
• ¾ teaspoon soy sauce
• ¾ teaspoon Worcestershire sauce
• ½ teaspoon Mexican oregano
• ¼ teaspoon pepper
• ¼ teaspoon kosher salt
• 4 (6- to 8-ounce) cod fillets, each cut lengthwise into 3 pieces
• 1½ teaspoons kosher salt
• ½ teaspoon pepper
• ¼ cup all-purpose flour
• 2 tablespoons vegetable oil, divided
• 2 tablespoons unsalted butter, divided
• 1½ cups finely shredded red cabbage
• 1½ cups finely shredded green cabbage
• 12 corn tortillas, warmed
• 1 large tomato, chopped fine
• ¼ cup coarsely chopped cilantro leaves and stems
• 1 avocado, sliced thin