In a large skillet cook the leeks, mushrooms, garlic, and rosemary in 1 tablespoon hot oil until leeks are tender but not brown. Meanwhile, bring a large pot of lightly salted water to a boil; add potatoes. Return to boiling; reduce heat. Cover and simmer for 3 minutes. Drain. (Potatoes will not be tender.)
Grease a 1-1/2-quart souffle dish or casserole. Arrange 1 cup of the potato slices over the bottom of the dish, overlapping slices if necessary. Spoon one-third of the leek mixture (about 2/3 cup) over potatoes. Sprinkle with 1/4 cup of the Parmesan cheese. Repeat layering twice. Drizzle top layer with 1 tablespoon oil. Bake, uncovered, in a 400 degree F. oven for 35 to 40 minutes or until potatoes are golden brown and tender. Let stand 10 minutes. Serve with yogurt or sour cream. Makes 4 servings.