Pre-soak the fish by placing the fish fillets in a shallow dish, pour the lemon juice on top, sprinkle ¼ teaspoon salt and turn the fish until well coated. Let soak for half an hour, turning once or twice. Move the fillets to a colander, discarding the juice, and rinse well. Air dry or pat dry with paper towels.

While the fish is soaking, make the agristada sauce. In a medium saucepan, whisk the eggs until evenly blended. In a small bowl, vigorously mix the warm water with the flour, until there are no visible lumps, then strain it through a sieve or a strainer into the saucepan. Add the oil, lemon juice and salt and place on medium-low heat, stirring constantly with a wooden spoon, until the sauce thickens, about 10 to 12 minutes. When it is about to boil, remove from heat, keep stirring for 1 more minute, then strain again into a glass dish. Taste and add more lemon juice or salt if necessary. Cover with plastic wrap on the surface of the sauce to avoid a film on top. Cool to room temperature, or in the fridge if not serving yet.

To fry the fish fillets, heat oil that reaches halfway up in a large pan (the oil is ready for frying when a wooden spoon handle placed in the oil results in small bubbles around it).

Mix the flour in a shallow bowl with ¼ teaspoon salt, and whisk the eggs in another shallow bowl with the remaining ¼ teaspoon salt.

Cover each fish piece in flour from all sides, shaking off excess flour, then dip in the eggs from all sides, letting the excess drip. Place a few pieces of fish in the oil, not overcrowding the pan, and fry on medium heat until well browned, 5 minutes on each side. Remove to a paper towel lined deep dish and continue with the rest of the fish in batches. You can keep the fish warm in a 220F oven, until the frying is done. Serve immediately with agristada sauce.