1. For crust, mix walnuts, zwieback, and margarine or butter. Press onto the bottom of a 9-inch springform pan.
2. For filling, in a large bowl beat feta and ricotta cheeses with an electric mixer on medium speed until well combined. Add eggs all at once, beating on low speed until just combined. Stir in mushrooms, chopped olives, milk, pepper, and salt with a spoon.
3. Pour filling into the springform pan, and place on a larger shallow baking pan. Bake in a 325 degree F oven for 40 to 45 minutes or until the center appears nearly set when shaken. Cool for 15 minutes on a wire rack. Run a spatula around inside of the pan. Cool for 30 minutes more. Remove side of pan. Cover and refrigerate at least 3 hours.
4. At serving time, garnish cheesecake with sliced ripe olives and fresh oregano, as desired. Serve with fresh sliced vegetables, if desired. Makes 20 appetizer servings.