6-8 large Yukon gold potatoes, about 2 pounds
1 medium sweet onion, finely diced
2 lemons, juiced
1/2 cup vegetable oil
2–4 tablespoons yellow ají paste (jarred), to taste
Salt and pepper
2 cans tuna (good quality)
1/2 cup mayonnaise (enough to hold the tuna salad together)
Boil potatoes in a large pot of salted water until tender, 20–30 minutes. Drain and, once cool enough to handle and in a large bowl, run potatoes through a ricer (or mash by hand, leaving no lumps).
Add onion, lemon juice, oil, ají paste, salt and pepper to the mashed potatoes. Mix thoroughly (with your hands or a large wooden spoon). Taste to check seasonings. Keep in mind that the heat of the ají and the other seasonings, including salt, will mellow as the dish refrigerates. It should convey some heat but not overwhelm. Set aside.
[Alternatively, you can add the onion to the tuna salad, next step.]
Drain tuna and mix with mayonnaise. Set aside.
Thinly slice avocadoes and toss with a little lime juice to hold their color. Set aside.
Lightly oil a 9×13 glass dish and line with plastic wrap, with some overhang on the edges. Evenly spread one-third of the potatoes into the pan. Cover with the tuna mixture. Layer another one-third of the potatoes over the tuna; cover with the avocado slices. Finish by layering the remaining potatoes over the avocadoes and smooth the top.
Cover with plastic wrap and chill thoroughly, for 3–6 hours.
To serve, invert the pan on a platter and remove the plastic wrap. Slice into squares and enjoy. Note: this is often served on a lettuce leaf garnished with quarters of hard-boiled eggs and Peruvian purple/black botija olives on the side.
*While tuna is traditional, the filling can be as varied as the person making the causa. You can do a crabmeat salad, one with cooked vegetables (e.g., corn, carrots and peas) or spread a little mayo on the layer and cover with poached fish or shrimp before spreading the potato layer on top.