Heat 2 tbsp of the oil in your Instant Pot on the sautee setting. Season the chicken with 1/4 tsp of salt and a pinch of pepper. Sautee the chicken in the IP for 3-4 minutes per side or until golden brown. Set aside.
Wilt the spinach in the hot IP. This should only take a few minutes. Set it aside and when cool, squeeze to remove excess water.
Heat the remaining tbsp of oil in the IP. Add the sliced onion and garlic and sautee for 2-3 minutes or until soft and aromatic.
Return the chicken to the IP and add the broth. Put the lid on and set to 10 minutes on the Meat setting. Cook, then let it release naturally for five minutes before manually releasing.
Remove the chicken from the IP and set aside. Put the IP back on sautee and simmer the cooking liquid for 3-4 minutes or until reduced by about a quarter. Add the heavy whipping cream and simmer another 3-4 minutes then turn off heat.
Add the spinach, Parmesan and Dijon mustard to the liquid. Stir well to combine. Season with salt and pepper if needed, and ladle generously over the chicken. Enjoy!
Garnish with some sliced green onions and fresh tomato for a pop of color and flavor!