Remove the stems from the mushrooms by carefully plucking them away from the tops. Place one mushroom cap on a cutting board and slice it with a large sharp knife. The slices should be thin, about an eight of an inch (or just used pre-sliced mushrooms). Set the slices aside then dice the onion. Cut the ends off the onion and remove the outer layers, then chop the onion into smaller pieces, about 1/4 of an inch thick. Set aside with the mushrooms.
Plug the Instant Pot in, making sure the lid is removed and the metal insert is inside. Press the Saute button and pour in 1/2 tbsp of oil into the metal insert. Let it warm up while you season the chicken thighs. Lay the chicken thighs out in a single layer on the cutting board. Sprinkle 1/2 tsp kosher salt and a bit of freshly ground black pepper (roughly 1/4 tsp) across the thighs. Take a pair of tongs and flip each thigh over and repeat on the other side.
Once the metal insert has warmed up and the oil is hot, use the tongs to carefully place the seasoned thighs in a single layer in the Instant Pot. The hot oil will splatter so you may want to wear a kitchen glove or wrap a towel around your hand. Let the thighs cook for 3 minutes, uncovered (i.e. don't put the lid on the Instant Pot), then flip them over with the tongs and cook for another 3 minutes. Remove them from the Instant Pot when done and set aside on a bowl or plate. (don’t set on cutting board as that has raw chicken juices on it).
Pour the remaining 1/2 tbsp of oil into the Instant pot, then dump the diced onion and sliced mushrooms into insert. Sprinkle the garlic powder and the last 1/2 tsp of kosher salt onto the veggies and stir them around with a wooden spoon. Let them cook for 3 to 4 minutes. Any browned bits that were on the bottom of the insert from browning the chicken will start to come off as the veggies start to sweat (i.e. release any water they have naturally).
As the veggies cook, measure out the rice and rinse it well with water to remove any loose starch that may be clinging to the outside of the grains. A fine mesh colander works well for this, or you can place the rice in a bowl, fill the bowl with water, then strain the water out.
Once the veggies have cooked down, press the Keep Warm/Cancel button to turn off the heat. Pour in the chicken broth, rice, and dried thyme into the Instant Pot. Juice the lemon and pour the juice into the insert as well. Stir slightly to combine and let rice and veggies settle under the broth. Grab the seared chicken thighs with the tongs and place them in the Instant Pot, putting them on top of the rice and veggie mixture but still sitting in the broth.
Place the lid securely on the Instant Pot, making sure the plastic ring is in place on the inside of the lid. Set the valve (the little knob on the top) on the lid to Sealing, then press the Manual button and set the time to 5 minutes by using the plus and minus buttons. The Instant Pot will take a while to come to pressure (roughly 5 to 10 minutes), then the display will turn to a number once it’s been pressurized.
When the timer goes off and the Instant Pot is done cooking the timer will go off. Unplug the Instant Pot and let it sit for 5 minutes (this is a very short “natural pressure release” where the ingredients keep cooking put the pressure starts to come down). Carefully turn the valve on the top to Venting, making sure to keep your hand and arm away from the top so as not to get hot steam and burn yourself. This is called a “quick pressure release”. When the pressure is fully released (you’ll hear a small click), remove the lid from the Instant Pot. Remove the chicken and stir the rice up a bit, then plate everything together and serve!