Combine salt and pepper and sprinkle over the chicken breasts.
Set Instant Pot to saute and add 1 tablespoon of olive oil. Cooking in batches place chicken breasts in the Instant Pot and brown on both sides. Approximately 10 -12 minutes. Set browned chicken aside.
Add the second tablespoon of olive oil to the pot and add the mushrooms, cooking for approximately 7 minutes until they are tender. Press cancel to turn off the saute function.
Pour stock into the Instant Pot, and deglaze the bottom of the pan, getting up all of the browned bits. Add the garlic, oregano, thyme, and rosemary and then place the browned chicken back in the pot.
Lock the Instant Pot lid in place and turn the valve to sealing. Set to high pressure for 8 minutes.
Once the timer goes off press cancel and let the Instant Pot do a natural release for 5 minutes before turning the valve to venting to release the remaining pressure.
Remove the chicken from the Instant Pot and cover with foil to keep warm.
Make a slurry by combining the cornstarch and water and stirring until there are no lumps.
Press saute and add the heavy cream, and dijon mustard. As the mixture heats slowly pour in the cornstarch slurry stirring constantly. Continue to cook for 5-7 minutes until the sauce thickens.
Add the chicken back to the sauce and serve with white rice.