Make the sauce
Combine jalapeños, ají amarillo, cilantro, garlic, mayonnaise, sour cream, lime juice, and vinegar in a blender. Blend on high speed, scraping down as necessary, until smooth. With the blender running, slowly drizzle in olive oil. Season to taste with salt and pepper. Sauce will be quite loose at this point but will thicken as it sits. Transfer to a sealed container and refrigerate until ready to use.
Prepare the pollock
In a large mortar and pestle, pound garlic and salt until a smooth, sticky paste forms. Add cumin, paprika, black pepper, and vinegar and grind them together to form a paste. Drizzle in vegetable oil while grinding. Alternatively, mince all ingredients with a knife and form into a paste. Cut the fillets in half, so you have 8 pieces total. Transfer fish and marinade to a large bowl and massage very gently with your hands until fish is completely coated in the marinade. Set aside while you heat your grill pan.
Grill the pollock
Heat a grill pan over medium-high heat (or set an electric griddle to 400F). Spray with nonstick cooking spray. Place fish on grill pan and cook about 4 minutes. Flip and cook on second side until cooked through and slightly browned, about 2 minutes more. Carefully transfer to a large plate.
Assemble sandwiches and serve
Place buns cut side down over the of the grill pan and cook, moving and rotating occasionally, until well toasted and beginning to char, about 1 minute. Transfer to a large cutting board. Use a fork to mash half an avocado on each bottom bun. Sprinkle with a little salt. Top with red onions, lettuce, and 2 pieces of fish each. Spread top buns with sauce and close sandwiches. Serve immediately, with any extra green sauce on the side.
Note: For a less spicy sauce, remove the ribs and seeds of the jalapeños before puréeing. Ají amarillo is a Peruvian yellow pepper paste that can be found in most Latin markets. It can be omitted.
3 whole jalapeño chilies, roughly chopped (see note)
1 tablespoon aji amarillo pepper paste (optional, see note)
1 cup fresh cilantro leaves
2 medium cloves garlic
½ cup mayonnaise
¼ cup sour cream
2 teaspoons fresh juice from 1 lime
1 teaspoon distilled white vinegar
¼ cup extra virgin olive oil
Kosher salt and freshly ground black pepper
Wild Alaska Pollock
3 medium cloves garlic, roughly chopped
4 teaspoons kosher salt
2 tablespoons ground cumin
2 tablespoons paprika
1 teaspoon freshly ground black pepper
2 tablespoons white vinegar
2 tablespoons vegetable or canola oil
4 wild Alaska pollock fillets (4-6 ounces each)