Season the chicken with salt and pepper.
Heat oil in a large sauté pan over medium heat. Sauté the chicken for four minutes on each side. Add the mushrooms, chopped sage, and the garlic; sauté for another three minutes, or until mushrooms are tender.
Add the sake. Bring to boiling. Reduce heat and cook for 2 minutes. Add the chicken stock; return to a boil. Reduce heat and cook for 4 to 5 minutes more. As the liquid starts to reduce, during the last two minutes, stir in the butter and remove from heat.
Spoon some of the mushroom mixture and cooking liquid into a shallow dish and top with a chicken breast. Garnish with fresh sage leaves, if desired.