1 (5 to 6 pound) roasting chicken

Kosher salt

Freshly ground black pepper

1 large bunch fresh thyme, plus 20 sprigs

1 lemon, halved

1 head garlic, cut in half crosswise

2 tablespoons (1/4 stick) butter, melted

1 large yellow onion, thickly sliced

4 carrots cut into 2-inch chunks

1 bulb of fennel, tops removed, and cut into wedges

Olive oil