Cook Fish-fillets, steaks or whole, 10 minutes for every inch of thickness. Measure at the thickest part. A 3/4-inch thick fillet requires about 7 to 8 minutes of cooking. a 1 1/2-inch thick whole trout will be done in about 12 to 15 minutes. Probe fish with a fork. A fillet is done when flesh has turned opaque at the thickest part. When cooking whole fish, probe with a fork to check the flesh around the bone. When it is opaque (no longer raw-looking) and easily pulls away from the bone, the fish is done.