Marinating frozen fish requires some extra care to ensure it's safe and results in flavorful fish. You're right; you should avoid letting the fish thaw at room temperature for an extended period, as it can lead to spoilage and foodborne illnesses. Instead, you have a couple of options for marinating frozen fish:
Method 1: Thaw and Marinate
Thaw the fish: The safest way to thaw frozen fish is in the refrigerator. Place the frozen fish in a bowl or a sealed plastic bag and allow it to thaw in the refrigerator overnight or for a few hours, depending on the thickness of the fish. Thawing in the refrigerator keeps the fish at a safe temperature, and any excess water from the thawing process can be drained off.
Pat dry the fish: Once the fish is fully thawed, remove it from the refrigerator and pat it dry with paper towels to remove any excess moisture.
Marinate the fish: Place the dried fish in a shallow dish or resealable plastic bag and pour the marinade over it. Ensure the fish is evenly coated in the marinade.
Marinate for the desired time: Depending on the thickness of the fish, you can marinate it for 15 to 60 minutes, but avoid marinating for too long as the acid in the marinade can start to "cook" the fish and affect its texture.
Method 2: Quick-Marinate without Thawing
If you prefer to skip the thawing step, you can use the quick-marinating technique:
Prepare the marinade: Prepare the marinade as you would for fresh fish.
Create an ice-water bath: Fill a large bowl with ice and water.
Seal the fish and marinade in a bag: Place the frozen fish and the marinade in a resealable plastic bag. Try to remove as much air as possible from the bag before sealing it.
Quick-marinate the fish: Submerge the sealed bag with the frozen fish and marinade in the ice-water bath. The cold temperature will help the fish thaw more quickly while also allowing it to marinate.
Monitor the thawing process: Keep an eye on the fish to ensure it doesn't thaw completely, as the goal is to thaw it just enough to absorb the flavors of the marinade. Depending on the thickness of the fish, this process may take 20 to 30 minutes.
Pat dry and cook the fish: Once the fish has partially thawed and marinated, remove it from the bag, pat it dry with paper towels, and proceed to cook it as desired.
Remember that marinating frozen fish should be a quick process to minimize the risk of bacteria growth. It's best to cook the fish immediately after marinating. If you prefer a longer marination time, it's safer to thaw the fish in the refrigerator first.