Preheat oven to 475°. Melt butter in small bowl; add minced garlic, chopped rosemary, the zest of the lemons and salt and pepper to taste. Set aside.
Place vegetables in bottom of Dutch oven. Reserve some of the onions for stuffing into the cavity. Prepare chicken by removing giblets from cavity, rinsing off and patting dry. Liberally season the cavity with salt and pepper. 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 veggies. Stuff the cavity with quartered lemon, onion, remaining garlic halves and rosemary sprig/stems. Tie the legs of the chicken together tightly using kitchen twine. 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°. If you want to, baste the chicken approximately halfway through cooking process.
Allow to sit for 10 minutes before you dig in.