Papi's Chicken with Calvados and Sautéed Apples

4 chicken breasts, each about 6 ounces / 170 g
2 tablespoons butter
1 tablespoon olive oil, plus more as needed
8 ounces (225 g) mushrooms, champignons de Paris or button, sliced into 1/4-inch / 6 mm pieces
2 medium shallots, finely chopped
⅓ cup (80 ml) Calvados or brandy
2 Gala or Golden Delicious unpeeled apples, cut into wedges
1 1/2 cups (360 ml) chicken stock
⅓ cup (80 ml) crème fraîche
salt and freshly ground  black pepper
golden crust potatoes, for serving