Turn the instant pot on sauté mode and add the oil. Once hot, add the diced onion and sauté for about 3-5 minutes or until fragrant and translucent.
Add the remaining ingredients except the flour and spinach. Stir to combine.
Place the lid on the instant pot and cook on high pressure for 15 minutes, with a 10 minute natural release.
Remove the chicken from the pot and let rest for about 5 minutes.
Meanwhile, turn the saute mode back on. Whisk in the flour until the sauce has thickened, about 2 minutes. Turn the sauté mode off and stir in the spinach.
Slice the chicken and add it back into the pot. Stir into the sauce.