Preheat oven or pellet grill to 425 degrees.
Add butter, garlic, rosemary, and thyme to a small saucepan and melt together over low heat. We are just melting the butter, no need to heat it up beyond this point.
Once the butter is melted, add the lemon zest and then the juice of the lemon to the mixture and stir to combine
Prepare the leeks by cutting off the root ends and the loosely packed green tops. All you want is the whiter, more tightly packed portion of the leek. Now, split it down the middle, lengthwise. Hold the leeks under running water and clean out any sandy soil by separating the inner layers of the leek and allow the water to run between them.
Prepping the chicken: make sure that all packaging is removed from the chicken, including the giblet pack that may be stuffed into the chest cavity of the chicken.
Now, using your fingers, carefully separate the skin from the breasts, entering from the tail end of the breasts. The skin should not be removed, we just want to create a separation between the two, to get seasonings directly onto the breast meat.
While holding the skin up and away from the breast meat with one hand, grab a finger-full of the butter herb mixture with the other. Now place the butter mixture under the skin and rub all over the breast meat. Work the butter all around the breasts. Try to get some up under the skin of the thighs as well. We want maximum coverage, directly onto the meat.
Once the meat is fully covered, rub the remaining butter mixture onto the surface of the skin and even add a bit to the interior of the chest cavity.
Season the chicken all over with sea salt and pepper. Try to get some under the skin and in the chest cavity. Then evenly coat the outside of the skin. Don’t go too heavy if you already have quite a bit of seasoning under the skin.
In the bottom of a rimmed baking sheet, arrange the prepared leeks into rows, about one inch apart.
Lay the chicken directly on top of the leeks and place into the oven/pellet grill.
Cook for about 1 hour to 1 hour and 15 minutes. Depending on the size of the chicken, you may go longer or shorter. You want a deep golden color on the skin, while also hitting a safe internal temperature. If the skin is getting too dark before the chicken is fully cooked, you can lower the heat until a proper internal temperature is reached.
Once an internal temperature of 155-160 degrees in the breast, and a minimum of 175-180on the thigh is reached, remove the chicken from the oven.
Allow the chicken to rest for at least 15 minutes before cutting. This allows the meat to relax and not squeeze all the juices out during premature cutting. The chicken will also continue to cook to a higher temperature for a minute or two after removing from the oven.