In a large nonstick skillet, heat 1 tablespoon olive oil. Add the spinach, season with salt and cook until wilted, about 2 minutes; drain. Wipe the skillet clean.
In a saucepan, bring 2 cups chicken broth and 1/2 teaspoon salt to a boil. Add the couscous, cover and remove from the heat; let stand for 5 minutes. Fluff with a fork.
Meanwhile, place the flour on a large plate. In a large, shallow bowl, beat the eggs and season with salt and pepper. In the reserved skillet, heat 2 tablespoons olive oil over medium-high heat. Working in 2 batches, coat the fish with flour, then with egg; transfer to the skillet and cook, turning once, until golden, about 5 minutes. Add 1 tablespoon olive oil between batches.
Using the same skillet, heat the remaining 2 tablespoons olive oil over medium heat. Add the onion and cook until softened, about 5 minutes. Add the remaining 1 cup chicken broth and simmer for 5 minutes; stir in the lemon juice and season with salt and pepper. Divide the couscous among 4 plates, top with the spinach and fillets and spoon the pan sauce on top; serve with the lemon wedges.