In a small bowl, combine the sage, thyme, chives, ¼ cup (60 ML) oil, and a pinch of salt. Stir into a paste. Set aside.
Generously season the chicken all over with salt and pepper. Then, using your fingers, gently release the skin over the chicken breast so that your fingers can slide between the skin and the breast meat. Spread half of the herb paste evenly between both breasts, then massage the skin with your fingers to push the herb paste to the places it didn’t reach. Reserve the remaining herb paste for later. Place 1 quartered lemon inside the cavity, then tie the legs together with kitchen twine. Tuck the wings under the chicken.
To a 6-quart Instant-Pot, add the onion, remaining 2 lemons, and the garlic. Place the chicken, breast-side up, on top, then pour in the chicken stock and sherry. Brush the remaining herb paste over the chicken. Cover with the Instant Pot lid, making sure the release valve is set to “sealing.” Set the Instant Pot to high pressure cook for 25 minutes.
Release the pressure from the Instant Pot by switching the vent from “sealing” to “venting,” making sure your hand is not over the top of the release switch, as the hot steam will come out of the top. Wait for the pressure to release and the Instant Pot to unlock before taking off the lid.
Transfer the chicken from the Instant Pot onto a baking sheet. If you would like crispier skin, set the broiler to high. Roast the chicken under the broiler for 3-4 minutes, or until the skin crisps and becomes a deep golden brown.
While the chicken skin crisps, turn the Instant Pot to high sauté mode. Bring the liquid in the pot to a boil and reduce by half, 5-6 minutes.
Add the cornstarch to the water and whisk together until smooth. When the sauce is reduced, whisk in the cornstarch slurry and continue to boil the gravy for another 2 minutes, or until the gravy is thick and coats the back of a spoon.
Strain the gravy through a fine mesh sieve.
Cut the chicken off the bone, place on a platter, and garnish with chives. Serve with the gravy.