A dish of pre-Hispanic origin, causa—sometimes made with crab or camarones instead of tuna—is often served as a lunchtime appetizer.
4 lbs. boiling potatoes
1/4 cup vegetable oil
1/4 cup extra-virgin olive oil
6 tbsp. freshly squeezed lime juice
4 tsp. rocoto paste (hot red chile paste)
2 tbsp. finely chopped white onions
3 6-oz. cans oil-packed tuna, drained
1 cup mayonnaise
2 avocados, peeled and cut into 1'' cubes
8 hard-cooked eggs, peeled and sliced
4–6 Greek-style black olives, pitted and halved lengthwise
1. Place potatoes in a medium pot. Cover with salted water and simmer over medium-high heat until tender, about 25 minutes. Drain, reserving 3/4 cup cooking water.
2. When potatoes are cool enough to handle, slip off skins. Place potatoes in a large bowl, gradually add reserved cooking water, and mash. When potatoes are smooth, stir in vegetable oil, olive oil, and 3 tbsp. lime juice. Beat until fluffy, then season with salt, add rocoto paste, and mix well.
3. Place onions in a small sieve. Rinse under cold running water to remove bitterness. Transfer to a medium bowl and add tuna, 2 tbsp. lime juice, and 1/2 cup mayonnaise. Mix thoroughly, then season with salt.
4. In a separate bowl, combine avocados with remaining 1 tbsp. lime juice and 1/2 cup mayonnaise. Season with salt, then mix gently.
5. To assemble, arrange one-quarter of the potatoes on a large plate. Cover with all the tuna mixture and another layer of potatoes. Place half the eggs on the potatoes, top with avocado mixture, then cover top and sides with remaining potatoes. Cover causa with plastic and refrigerate for at least 1 hour. Garnish with olives and remaining eggs.