Preheat oven to 425 degrees F.
Dry chicken and it's cavity with a paper towel. Season the cavity with 1 teaspoon salt and 1 tsp, then stuff the bird with half the onion, the celery stalks, both halves of the garlic head, the juiced lemon (not the juice!) and quartered lemon.
In a 9x11 baking dish, pull apart the layers of the other half of the onion, and place them in one layer in the baking dish. Place the chicken on top of the onion.
In a small bowl, mix the butter, lemon juice, dijon mustard, oregano and rosemary together until well combined. Make pockets beneath the chicken breast skin with your hands, then rub ⅓ of the butter mixture beneath the skin of the breasts, over the entire breast generously. Use the rest of the butter mixture to rub onto the skin of the chicken, evenly into every nook and cranny. Make sure to get butter into the crevices of the legs and wings as well.
Salt and pepper the chicken well over the entire bird, making sure to get into crevices. Tuck the wings under the bird, then place into the oven. Let cook at 425 for 30 minutes, then turn temperature to 350 degrees F for 1 hour -1 hour and 15 mins, or until internal temperature is 155-160 degrees F. Temperature will rise as the bird rests.
After chicken is done cooking, let rest for 15 minutes before serving.