Preheat oven to 475. Melt butter in small bowl; add minced garlic, chopped thyme, the zest of the lemons, and salt and pepper to taste. Set aside.
Thickly slice the onions and place in bottom of enameled cast iron dutch oven. Reserve the end scraps. Prepare chicken by removing giblets from cavity, rinsing off, and patting dry (a lot of chicken come with them already removed!). Liberally season the cavity with salt and pepper.
Cut the lemons in half; reserve juice and save rinds.
The butter mixture will have hardened slightly by now. Using your hands, slather the mixture liberally over every part of the chicken and under the breast skin (being careful not to tear the skin). Place in dutch oven on top of sliced onions.
Stuff the cavity with the lemon rinds, onion scraps, remaining garlic, and thyme sprig/stems.
Tie the legs of the chicken together tightly using kitchen twine. Pour reserved lemon juice over the chicken. Roast chicken for 15 minutes at 475, then reduce heat to 350 and cook for approximately 20 minutes per pound, or until internal temperature reaches 165 degrees.
* Note-You should cook the chicken uncovered in the dutch oven, but if it starts to burn on top and the internal temperature is not high enough yet, you can place the lid on ajar to avoid direct heat and prevent burning.