Preparation
Season the fillets with 2 tablespoons of the lemon juice, and 1/2 teaspoon of the salt and let stand 5 minutes.
In a large skillet, heat the oil until hot.
Add the fillets and fry them lightly until just cooked through. Let cool, then chill.
Peel and pit the avocado. In a bowl mash the avocado, add the remaining 2 tablespoons lemon juice, the olive oil, 1/2 teaspoon salt, onion, and parsley, and combine well.
Line a serving platter with the sliced potato.
Place the fish fillets in the center and pour the avocado puree over everything.