Place a fillet on the flour, pat it down, turn it, and pat it on the other side to thoroughly coat the fillet with a thin layer of flour. A piece of fish needs to be heated to 145º or higher to be safe for consumption. Adjust gently, but only for a few seconds. The trick is to retain an even heat, so that all parts of the base of the fish cook evenly. 5. Cook the fish for about 90 seconds and turn, taking care. Cook the same on … Shake off any excess flour. The best way to tell if a piece of fish is thoroughly cooked is to check the temperature with a meat thermometer. If the fish is coated in flour or breadcrumbs, cook until the coating is a golden-brown. For an extremely lightly coated fish, you can go directly to Step 7 to Cook the Fish Fillets. Carefully lift the fish out of the pan with the spatula. The entry of the fish will drop the temperature of the pan a little.