Perfect Roast Chicken

(5 to 6 pound) roasting chicken

Kosher salt

Freshly ground black pepper

large bunch fresh thyme, plus 20 sprigs

lemon, halved

head garlic, cut in half crosswise

butter, melted

large yellow onion, thickly sliced

carrots cut into 2-inch chunks

bulb of fennel, tops removed, and cut into wedges

Olive oil