1. Brown beef and drain.
2. Stir in tomato sauce, undrained mushrooms, oregano, and garlic salt.
3. Bring to boil and remove from heat.
4. Combine macaroni and milk.
5. In a greased 2-quart casserole dish, layer half each of the meat sauce, macaroni mixture, cheese, and pepperoni.
6. Repeat layers, saving a little meat sauce for top.
7. Garnish with remaining pepperoni slices.
8. Bake (covered) at 350°F for 25-30 minutes.