In a small pot of water, boil the potatoes for about 10 minutes or until almost cooked through. Set aside to cool and then slice them into 1/4 inch rounds. Spread them across the bottom of an oiled baking dish.
Preheat your oven to 400 degrees Fahrenheit.
Sprinkle all of the peppers, tomatoes, onions, and garlic over the top of the potatoes. Squeeze the half a lime over the top and drizzle the entire thing with olive oil. Season with salt and pepper and then break apart the butter with your fingers and scatter the pieces over the top of the vegetables.
Mix the Ají Amarillo paste with the mayonnaise.
Place the cod fillet over the top of the vegetables and slather the mayonnaise mixture over the top of it. Season the fish with salt and pepper.
Bake until the fish is cooked through and the vegetables are softened but not mushy, about 15 to 20 minutes.
Garnish with cilantro, basil, green onion and Serrano pepper rounds.