Wash the potatoes and slice in half lengthwise. Put in a large mixing bowl. Drizzle the oil over the top and toss well to coat.
Wash and dry the sage leaves, fold in half, and cut away the thick rib on the back. Mince finely and add to the bowl. Sprinkle with the salt and pepper and toss well to coat.
Spray a baking dish and add the potatoes. Bake for 30 minutes, stirring once after 20 minutes. Taste one. It should split easily with a fork, and be soft inside and crispy and golden on the outside. Serve with mushroom gravy.