Squeeze the juice of lemons into a measuring cup. Add an equal amount of olive oil. Add the garlic, salt, and pepper. Stir well.
Rinse the cusk under cold water and pat dry. Place the cusk pieces in a single layer in the bottom of a baking dish. Pour the marinade over the fish, cover, and refrigerate for at least one hour. Grill the fish on medium-high heat. Baste cusk with marinade.
Cooking time is as follows: The thin end of the cusk, grill approximately 2 minutes per side. The thicker end of the cusk grill 4 to 5 minutes per side.