Preheat your oven to 300°F (150°C).
In a small bowl, combine butter with 1 tablespoon of the chopped sage. Rub half the butter mixture under the chicken skin, and the rest on the outside.
In a large bowl, toss the diced potatoes with olive oil, remaining sage, and a generous pinch of salt.
Spread the potatoes on a large sheet pan. Nestle the chicken pieces on top.
Season chicken with more salt if desired.
Roast in the oven for 25 minutes.
Increase the oven temperature to 450°F (230°C). Roast for another 15 minutes, or until chicken skin is crisp and golden.
If potatoes need more crisping, remove chicken and return potatoes to the oven for another 5–10 minutes, tossing once.
Let rest for 5 minutes before serving.