Cast-Iron Skillet Whole Roasted Chicken With Potatoes

to 4½ pound whole chicken

Kosher salt

Olive oil

Freshly ground black pepper

onion, any flavor will do, sliced

russet potato, very thinly sliced (no need to peel the skin)

sweet potato, very thinly sliced (no need to peel the skin)

sprigs of fresh thyme