Rinse, and pat dry the fish with a cloth or paper towel and season with salt.
Prepare the marinade with the lemon juice, onions, pepper, minced garlic, and stock cube.
Douse the fish with the marinade and refrigerate for at least an hour.
In a large pan, heat oil over medium heat. Add the fish, then cook on each side for about 5 minutes or until cooked through and crispy. Remove fish and set aside. Drain oil and leave about 2 tablespoons.
Fry the onions with the bay leaves and marinade for 5 minutes.
Add ½ cup (100ml) of water, cover and simmer for 5 minutes. Add salt and pepper to taste, then discard the bay leaves.
Add the fish back to the pan on top of the sauce. Simmer for another 8 to 10 minutes.
Place the fish on a plate, top with the lemon and mustard sauce over it. Garnish a lemon wedge and serve with white rice.