Season the chicken with extra salt and pepper (optional), then set aside while the Instant Pot gets hot.
Turn the Instant Pot on using the saute function. Allow the liner to get hot, then add the oil. Add the chicken breasts to the pot and cook for 3-4 minutes on each side.
Remove the chicken, and set aside.
Keep the Instant Pot on saute, and add the onions. Use a wooden spoon or spatula to stir the onions. Cook until soft, about 3-5 minutes, then add the garlic, salt, and pepper. Cook an additional 30 seconds. Turn off the Instant Pot.
Pour in the chicken broth and lemon juice. Stir to combine.
Add the chicken breasts back to the pot in a single layer. Top each with a fresh thyme sprig if using.
Close the lid, and set the valve to sealing. Use the manual (pressure cook) button and adjust the time to 8 minutes (per pound). After the time is up, allow a natural release for at least 10 minutes, more is better. Open the valve to release any remaining pressure.
Remove the chicken, and set aside
Combine the cornstarch and cold water to make a slurry. Add the slurry to the Instant Pot and turn on to saute. Whisk to combine and allow to thicken.
Serve the chicken in the sauce or drizzle the sauce over the chicken. Chicken can be served with rice, pasta, or steamed vegetables.
The calories listed are an approximation based on the ingredients and a serving size of four (4) ounces of chicken breast. Actual calories will vary. The lemon chicken can be stored in your fridge in a sealed container for up to three (3) days.