Season both sides of chicken with salt and pepper.
Turn your Instant Pot to sauté and add 2 tablespoon oil. Once hot, add the seasoned chicken and brown both sides of chicken. About 3 minutes per side.
Remove chicken from Instant Pot with tongs and set aside for a moment.
Add remaining oil to the Instant Pot. Once hot, add onions, mushrooms and garlic. Cook, stirring occasionally, until the mushrooms are tender. About 5 minutes.
Season with more salt and pepper if desired.
Turn off your Instant Pot. Add broth/stock and Italian seasoning. Use a wooden or long spoon to scrape and deglaze the bottom of the Instant Pot.
Add chicken back to the instant pot and lock lid into place and make sure valve is switched to sealing.
Cook on HIGH pressure for 5 minutes. If your chicken breasts are extra thick, consider cutting them in half or adding an extra minute or two the cooking time. It will take your Instant Pot about 7-10 minutes to get up to pressure
While chicken cooks, mix the tapioca/arrowroot and milk, whisking until lumps are gone. Set aside.
After cook time is over, quick release the pressure and carefully open the lid.
Remove the chicken and place on a cutting board or plate.
Add coconut milk mixture to the mushrooms and stir to combine.
Again, turn your Instant Pot to the saute function and let everything cook for about 5 minutes or until sauce thickens.
Taste and season as needed. If sauce is too thick, add a little extra broth to thin it out.
Add spinach. Stir a few times to incorporate and to slightly wilt the spinach in the hot sauce.
Slice chicken if desired and stir back into the sauce
Serve warm with rice, pasta, steamed/roasted veggies, baked potatoes or quinoa.