In a small bowl, mix together the salt, pepper and spices until evenly combined. Pat the turkey breasts dry with paper towels. Sprinkle both sides of each turkey breast with the seasoning mix and massage it inches.
Press the sauté function on a 6-quart or larger Instant Pot. Once hot, add the olive oil and brown the turkey breasts for about 3 to 4 minutes on each side, working with one at a time. Transfer the turkey onto a plate.
Turn the machine off and add in the chicken or turkey stock, stirring with a wooden spoon to scrape up all the browned bits from the bottom. Carefully place the wire steaming trivet on the bottom of the pot. Cut the onions, celery and carrots into large pieces and place them on the rack in an even layer. Nestle in the browned turkey breasts into the Instant Pot, you might have to stand them up to make them fit. Place the thyme and rosemary sprigs on top and then lock on the lid. Set the valve to seal and then pressure cook on high for 45 minutes. Allow the pressure to naturally release until the lid unlocks. Do not rush the release or else the turkey will dry out.
Carefully open the lid and transfer the turkey breasts onto a baking sheet, skin side up. If you want to crisp up the skin, you can place them under the broiler to brown them, but that’s optional. Cover loosely with foil to keep warm while you make the gravy.
Remove the steaming trivet and then carefully strain the stock into a large bowl through a fine mesh sieve. Discard the onions, carrots and celery. Set the bowl back into the Instant Pot and wipe it clean with a paper towel. Press the sauté function on medium and then add the butter. Once melted, whisk in the flour and cook for about 30 seconds or so. Then whisk in the strained stock and cook until the gravy has thickened. Season with a pinch of salt and pepper and the Worcestershire sauce. To serve, slice the turkey and pour the gravy over the top. Garnish with parsley.