1. Put the lemon juice and the chopped onion in a bowl to marinate; season with salt, pepper and ground chili.
2. Boil the potatoes with their skin. When cooked, peel and mash with a fork. Add the lemon juice and onion. To keep the mixture smooth add the oil gradually. Adjust seasoning if necessary.
3. Put the mixture in a mold. To serve unmold and garnish with shrimp tails, olives, hard-boiled eggs and cubes of fresh cheese.