I've used the confit method to successfully poach fish in oil, switching out the duck fat for olive oil. Preheat your over to 200 degrees F. Take a nice, meaty fillet of the fish of your choice (halibut, salmon, tuna, belly), salt it if you like, and put it in a deep, ovenproof vessel. Cover the fish with olive oil (by cover I mean "immerse"). The fish should be submerged in a bath of olive oil, and covered by at least a half inch, more is better. You needn't use your best extra-virgin olive oil. Inexpensive, light olive oil with a mild flavor is best in my book. Cover the vessel with foil and put it in the oven. The oil will slowly heat, gently cooking the fish from all sides and preventing it from drying out. It will need to stay in the oven for a while, but since I'm impatient I check frequently once the oil is up to temperature. With your thermometer, check for an internal temperature of 130-140 for halibut-type fish. For salmon and tuna, I like to stop at 120-130. Remove the fish gently from the oil (it will be very delicate) and serve. Once the oil cools, it can be well-strained, refrigerated, and re-used.