In a medium bowl, rub both fish inside and out with 1 tablespoon of the lemon juice, then season with the salt.
To a pot wide enough to hold the fish, add the oil and onions and turn the heat to medium-high. Cook, stirring frequently, until soft and golden brown, about 16 minutes.
Add the chile and 4 cups of water and bring to a boil. Add the fish, turn the heat to medium-low, cover, and simmer until the flesh is nearly opaque, about 10 minutes. Add the remaining lemon juice and continue cooking until the fish is fully cooked, about 5 minutes more. Season to taste with salt and serve hot over rice.
