Rainbow trout are indigenous to the western United States and Canada, but they’re bred and sold all over the world. They’re kin to salmon and their meat is mild and delicate. Pan-frying is an ideal way to prepare rainbow trout fillets as long as you don’t overdo it. If you cook them too long, the flesh will dry out and become less appetizing.
Turn a stove-top burner to medium-high heat and place a frying pan on it. Prepare your fillets for frying while it’s heating up. Pat them dry with a paper towel to remove any excess moisture, then dredge them in a mixture of flour, salt and pepper.
Add the vegetable oil and the butter to the hot frying pan. Two tbsp. of oil and 1 tbsp. of butter are enough for two 8-oz. fillets; if you’re going to be preparing more than that, do it in batches and add a little more oil and butter with each batch.
Place your fillets skin-side down in the frying pan as soon as the butter has melted. Act quickly so the butter doesn't burn. Pick the pan up and shake it gently from side to side so the fish doesn’t stick to it. Replace the pan to the burner and cook for three to five minutes until the underside begins to turn to a golden crust.
Flip your fillets over gently to cook the other side. Pick the pan up and shake it again. Cook for another two to three minutes, then begin watching for doneness. Your rainbow trout is done when a fork or toothpick inserted in the center of the fillet reveals moist, opaque flesh. If the meat has a translucent look, cook your fillets a little more, but check it again frequently to avoid overcooking.
Transfer your fillets to a serving platter. If you’re making another batch, you can place the platter in the oven at very low heat, approximately 150 degrees F, while you prepare the remaining fillets.