Mix all the ingredients in a bowl. Separate into 2 equal lumps and roll them to create a cylinder about 10 cm in length, then wrap them in a plastic film. Leave in the refrigerator until you need them. Freeze the remainder after use.
Use an absorbing paper towel to dry off the fish after it’s been thawed. Spread the flour in a plate and dip both sides of the fish to coat them. Season with salt and pepper to taste, and then set aside. In a pan over medium high heat, break one half of a butter cylinder, then grill the fish, about 3 minutes on each side. Serve with a garden salad.