In a small bowl, combine the onion and lemon juice and season with salt and pepper. In a medium saucepan, heat 1 tablespoon olive oil over medium heat. Add the garlic and cook for 1 minute. Add the chicken broth, season with salt, cover and bring to a boil. Stir in the couscous and parsley and remove from the heat; cover and let stand for at least 5 minutes before fluffing with a fork.
On a shallow plate, beat the eggs with 1 tablespoon water. In a large nonstick skillet, heat 1 tablespoon olive oil over medium-high heat. Working in 2 batches, season the fish with salt and pepper, then dip into the egg mixture, turning to coat; add to the pan. Cook, turning once, until golden, about 5 minutes; transfer to paper towels to drain. Repeat with the remaining 1 tablespoon olive oil and the remaining fish and egg mixture.
Divide the couscous among 4 plates. Arrange the fish on top and sprinkle with the pickled onions. Serve with the lemon wedges.