Preheat oven to 425°F (220°C). Pat the chicken completely dry inside and out. Mix the 2 tsp kosher salt, 1 tsp smoked paprika, 1 tsp garlic powder, 1 tsp onion powder, ½ tsp black pepper, ½ tsp dried oregano, and ¼ tsp cayenne in a small bowl. Rub the 2 tbsp olive oil all over the bird, then press the spice mixture onto every surface. Slide the 1 tbsp softened butter under the breast skin and spread it across both breasts.

Stuff the halved yellow onion into the cavity. Tuck the wing tips behind the back and tie the legs loosely with kitchen twine.

Place the chicken breast-side up on a rack in a roasting pan. Roast at 425°F (220°C) for 20 minutes, then reduce to 375°F (190°C) and roast for another 40 to 50 minutes until a thermometer in the thickest part of the thigh reads 165°F (74°C).

Rest the chicken on the rack for 10 minutes, then transfer to a carving board. Remove the twine and onion, then carve into pieces starting with the legs and thighs, spooning the pan drippings back over the meat.

Arrange the carved pieces skin-side up on a warm platter, spoon over the pan drippings, and garnish with fresh thyme or parsley sprigs.