Season the chicken with salt and pepper and let it sit in the fridge, uncovered, overnight.
Preheat the oven to 475F degrees.
Place the onions, garlic, potatoes and pancetta in the bottom of a Dutch oven (pancetta on top).
Gently separate the skin on the breast and legs from the meat and stuff with the 4 pats of butter.
Stuff the cavity of the chicken with the halved lemon, fresh thyme and fresh sage. Place in the Dutch oven.
Rub the bacon fat on the skin and pour the white wine over the chicken.
Roast, uncovered, for 15 minutes, until the skin has started browning.
Reduce the heat to 350F degrees and roast for an additional 2 hours, basting 1-2 times throughout.
The chicken should register 160F degrees when you remove it from the oven.
Let it rest for 10 minutes, carve, then serve. **Optional: squeeze the lemon from the cavity over top of the chicken.
While the chicken rests on a cutting board, place the Dutch oven with the potato mixture back in the oven at 500F degrees and cook until the potatoes caramelize, about 5-10 minutes.