Cook the potatoes in boiling salted water, peel them and pass them thru a potatoes ricer while still hot. (tip: you can get a better texture if let the potatoes dry 10 minutes before mash them)
Let coold down and then add the ají amarillo paste, vegetable oil (1 oz. to 2oz.) the lime juice and salt. Mix well with hands until you get a uniform and soft "dough". Taste it Add more ají if you like it spicy. The quantities are up to you depending on your taste. The secret is get a balance spicy, sour and salted. The oil will make the "dough" softer and easy to handle.
Put in a ring your first layer of potatoes "dough", put avocado slices on borders and add a second layer of the tuna on top, "close" your causa with another layer of potatoes
Afterwards, you can add avocado cut into fine slices (maybe some bell pepper strips) and seasonde with salt and pepper.
Cover with mayonnaise and finally cover with another layer of causa. Extend with your hands.
Garnish with some mayonnaise and parsley.
if you don't like tuna, try some chicken salad... the combinations are unlimited
If you don't have the rings try to make the causa like a three laters lassagna