Generously season chicken with salt and pepper and 1 teaspoon Italian seasoning/herbs de provence on both sides. Set pressure cooker to saute and melt 2 tablespoons of butter in the bottom. Drizzle with 2 teaspoons oil to keep the butter from burning.
Sear chicken in the pot for 2-3 minutes on each side. Remove chicken from the pot and set aside.
Melt remaining 1 tablespoon butter in the pot, stir in garlic and mushrooms for 1-2 minutes til garlic is fragrant. Stir in remaining 1/2 teaspoon Italian seasoning/herbs de provence.
Stir in chicken broth and marsala. Return chicken to the pot.
Put the lid on and slide into the locked position. Turn the vent to the sealed position.
Set to MANUAL or PRESSURE COOK on HIGH for 8 minutes.
Do a quick release by turning the valve to the venting position to release the pressure. Once the float valve drops, remove the lid and use a slotted spoon or tongs to transfer chicken to a plate and cover to keep warm.
Set pressure cooker to SOUP setting. Stir in 1 teaspoon salt and 1/4 teaspoon pepper. Whisk together cold water and corn starch til dissolved, then whisk into the pressure cooker. Once thickened, turn pressure cooker off. If using heavy cream, stir in now.
Pour the mushroom sauce from the pot over the chicken and serve.