Preheat oven to 475 degrees F.
Melt butter (4 tablespoons) in small bowl; add minced garlic (3 cloves), chopped rosemary (from 3 sprigs, about 2 tablespoons), the zest of the two lemons, and plenty of kosher salt and black pepper to taste. Set aside.
Place the onions in bottom of enameled cast iron dutch oven. Reserve the end scraps. If using potatoes, add them on top of the onions.
Prepare chicken by removing giblets from cavity and patting dry with paper towels.
Liberally season the cavity with salt and pepper.
Cut the zested lemons in half; reserve juice and save rinds.
Using your hands, slather the butter mixture liberally over every part of the chicken and under the breast skin.
Stuff the cavity of the chicken with the lemon rinds, onion scraps, remaining 2 garlic cloves, and rosemary sprig/stems. Place in dutch oven on top of sliced onions and potatoes, if using.
Tie the legs of the chicken together tightly using kitchen twine.
Pour reserved lemon juice over the chicken.
Roast chicken UNCOVERED for 15 minutes at 475, then reduce heat to 350 and cook STILL UNCOVERED for approximately 20 minutes PER POUND, or until internal temperature reaches 165 degrees.
Allow the chicken to rest for at least 10 minutes before carving.
