Fill the inner pot of your Instant Pot with water to the max line. Place the steamer basket inside the pot.
Place the lid on and press the sous vide function, adjust the temperature to 50°C. Press the sous vide button again and set the time to 1 hour. Wait for it to come up to temperature – it will beep and the countdown will begin once it is at the correct temperature.
While it is coming to temperature prepare your fish. Drizzle the fish portions with olive oil and season with salt and pepper. Place a slice of lemon in the middle of each portion. Carefully place the fish inside a vacuum sealed bag, freezer bag or stasher bag. If using a vacuum seal bag, seal completely. If using a freezer bag or stasher bag, remove as much air as possible and seal the bag.
When the Instant Pot has come to temperature, place the bag of fish inside, making sure that the top (where the seal is) is not submerged.
Place the lid on and allow the fish to sous vide for one hour.
While the fish is cooking prepare the gremolata. Add all ingredients to a food processor and pulse until you get a coarse mixture. Season to taste and set aside.
When the fish is finished, gently remove it from the instant pot and your bag. Discard the slices of lemon.
Heat olive oil in a large frying pan and sear the skin side of the trout for 1 minute, pressing it gently but firmly with a spatula. Remove and serve skin side up topped with gremolata.