Position rack in the middle of oven and preheat to 375 degrees F.
Butter a large baking dish or pan that will hold the chicken and potatoes in a single layer.
Rinse and pat dry chicken.
In a sauté pan over medium-high heal, melt 2 tbsp of butter with the oil. When foaming, add half of the chicken and quickly sear until golden, 4-5 mintues on each side. Transfer to the prepared dish. Repeat with the remaining chicken. Arrange the potato slices among the meat.
Reduce the heat to medium-low and add the shallots. Sauté until translucent, 2-3 minutes. Add half each of the apple juice and stock and stir, scraping up any browned bits. Bring to a boil and pour over the chicken and potatoes. Tuck the sage leaves around the chicken. Sprinkle with salt and pepper.
Bake until the potatoes and chicken are tender, 45-50 minutes. The meat should be opaque througout. Using a slotted sppon, transfer the chicken and potatoes to a serving dish and cover to keep warm. Using a large spoon, skim off the fat form the juice and pour the juices into a pitcher.
In a small saucepan over medium-low heat, melt the remaining 1 tbsp of butter. Add flour and cook, stirring, until bubbly, 1-2 minutes. Raise the heat to medium. Stirring constantly, gradually add the remaining 1/4 cup apple juice and stock and pan juices. Stir until the mixture thickens and comes to a boil, 3-4 minutes. Add the cream and cook for a few seconds. Thin, if desired, with apple juice or stock. Adjust seasonings to taste. Pour the sauce over the chicken and potatoes and serve.