Pre Heat oven to 400 degrees F (204 C)
Pat your chicken dry, using paper towel. Season the cavity well with salt and pepper (reserve some for the butter mixture
Cut your lemongrass lengthwise, and take a small slice to mince. Reserve the remaining lemongrass for stuffing.
Mince 2 cloves of garlic and add it to a bowl of your softened butter, together with the minced lemongrass. Add 1/2 teaspoon of pepper and 1/2 tablespoon of salt. Mix all together then spread evenly on your chicken. Rub it using your hands.
Pound the remaining lemongrass (to get the most flavor out) until it is bruised and it is easier to tie it into a knot. Then insert the tied lemongrass into the seasoned chicken cavity.
Cut half of 1 onion into quarters or whatever you prefer, and put it inside the chicken cavity, add the remaining garlic (smashed your garlic).
Place the remaining onions (cut into rings) and layer it into the pan, then put the chicken on top of the onions
Adjust your oven temperature to 475 degrees F (246 degrees C) and roast your chicken for 15 minutes, uncovered. Then turn the heat down and let it roast to 350 degrees F (190 degrees C) for an hour or more, depending on the size of your chicken until the internal temperature reaches to 165 degrees. Make sure when you insert your meat thermometer that it does not touch the chicken bone, when taking the chicken temperature, and the juices are running clear.
Let your roast chicken rest for 10-15 minutes. Tent it with foil, to keep it warm, before carving it. This makes the moisture in your chicken re-absorb all the juices and keeping it tender and juicy.