Mix 4 tablespoons butter, the chopped rosemary and thyme, lemon zest, garlic, 1/4 teaspoon salt and 2 grinds of pepper in a small bowl. Pat the chicken dry and generously season inside and out with salt and pepper. Gently loosen the skin from the meat on the breasts and thighs, and stuff the herb butter under the skin. Stuff the cavity with the onion, rosemary sprigs and lemon wedges. Tie the legs together with twine.
Put 1 cup water, the bay leaf and thyme sprigs in a 6-quart Instant Pot. Place the steam rack in the pot with the long handles positioned up to make it easy to pull out the chicken later. Place the chicken breast-side up on the rack. Put on and lock the lid; make sure the steam valve is in the sealing position. Set the pot to pressure-cook on high for 21 minutes. When the time is up, let the pressure release naturally, about 15 minutes. Turn off the pot and remove the lid. Carefully lift out the chicken on the rack and transfer the rack to a baking sheet to catch any juices. Let the chicken rest 15 minutes.
Meanwhile, make the gravy: Pour the cooking liquid into a measuring cup; discard the bay leaf and thyme sprigs. Spoon off as much fat as possible into a small bowl. Rinse and dry the inner pot. Set the Instant Pot to saute on high. When the pot registers hot, add the flour and 2 tablespoons of the reserved fat and cook, whisking frequently, until the flour turns caramel brown, 3 to 4 minutes. Gradually whisk in the reserved cooking liquid; continue whisking until there are no lumps. Bring to a boil, then simmer until thickened, about 5 minutes. Season with salt and pepper.
Preheat the broiler. Cut the chicken into pieces, cutting the breasts off the bone, and place on a baking sheet. Brush with the remaining 1 tablespoon butter and broil until the skin is browned and crisp, 3 to 4 minutes. Serve with the gravy.
Finish this chicken under the broiler to crisp up the skin or serve it straight out of the Instant Pot and just remove the skin.