On the Instant Pot, press on the SAUTE button and let the pressure cooker preheat. When the message HOT is displayed, add the oil and sear the chicken for 2-3 minutes. It does not need to be cooked all the way through.
Move the chicken to one side of the pot, then add the diced onion and bell peppers and cook them for a few minutes.
Switch off the SAUTE setting and add the diced tomato.
Rinse the rice until the water runs clear, then add it to the pot. Add the fajita seasoning with the stock and cover the pot with the lid.
Cover the Instant Pot with the lid and set the valve on SEALING position. Press on PRESSURE COOK or MANUAL on high for 8 minutes. Allow the Instant Pot to come to pressure, and cook. Once the cooking has finished, quickly release the steam.
With a fork, carefully fluff the rice.
Optional: Stir in sour cream, then sprinkle cheese on top and cover the Instant Pot again with a lid for 1 minute. The residual heat will allow the cheese to melt.
Give everything a quick stir and serve with your toppings of choice.