- Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain.
- Mash potatoes with a ricer. Gradually stir in oil until potatoes come together; add aji amarillo, key lime juice, salt, and pepper.
- In a separate bowl mix crab, onions, key lime juice, salt and pepper.
- Line a casserole dish with plastic wrap. Spread 1/2 the potato mixture on the bottom of the dish.
- Spread 2 tablespoons mayonnaise over the potatoes, spread the crab mixture over the mayonnaise, and place the avocado slices in a single layer on top of the crab mixture. Spread the remaining 1/2 of potato mixture over the avocados, and top with remaining 2 tablespoons mayonnaise.
- Cover casserole dish with plastic wrap and refrigerate until firm, about 30 minutes.
- Invert casserole dish onto a serving dish or baking sheet to remove potato casserole from dish.
- Remove plastic wrap and cut casserole into squares.