bring to a boil potatoes into a large pot and cover with salted water; Reduce heat to medium-low and simmer until tender, about 18 - 25 minutes.
Put aji amarillo (yellow chili) in a blender with 1/2 cup of water to blend and mix good until have a cream. Squeeze the 6 lemons into a cup. Set aside
Mash potatoes with hand or a ricer mixer until smooth. Little by little stir in oil until potatoes come together; add aji amarillo cream, salt and lemon juice, and pepper. Cool potato mixture in the refrigerator, about 20 minutes.
Stir tuna, diced onion and 1/4 cup mayonnaise together in a bowl.
Use a rectangular glass bowl. Drop a litlle oil and Spread 1/2 potato mixture on the bottom of the dish. Spread 2 tablespoons mayonnaise over the potatoes, then spread the tuna mixture over the mayonnaise, finally place the avocado slices in a single layer on top of the tuna mixture. After that Spread the remaining 1/2 of potato mixture over the avocados, and top with remaining 2 tablespoons mayonnaise.
Place sliced eggs over the top. Cover casserole dish with plastic wrap and refrigerate until firm, about 30 minutes.
Another variation consist in decorate with boiled shrimps and scallops avoiding boiled egg. Cut the causa in squares to serve in dishes, this is a delicious starter plate from Peruvian cuisine
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.