Like many other species of flatfish, halibut is so lean that it is really easy to overcook. When a meaty halibut fillet is cooked just right, it is succulent and flaky; when it's overdone, it becomes so tough that you might even need to grab a knife to get through it. Yuck! Halibut is cooked through when the fish flakes easily with a fork and appears opaque. If you’re not sure that the halibut is done, you can use a digital thermometer (like this one) to measure the internal temperature. I recommend checking for doneness after about 10 minutes of cooking. If the temperature is at least 120 to 130 degrees F, then the halibut dish is ready to eat.