Heat oven to 375°F. In large bowl, combine ground beef, bread crumbs, garlic, oregano, 1/2 teaspoon salt and egg; mix well. Divide mixture evenly into 8 ungreased muffin cups. Pat in bottom and up sides of each cup to form crust.
In medium saucepan, prepare potatoes as directed on package using water, milk, margarine and salt. Add feta cheese; stir gently to mix. Fill beef-lined cups with potato mixture.
Place cookie sheet on oven rack below muffin pan in case of spillover. Bake at 375°F. for 15 to 20 minutes or until beef is thoroughly cooked. Remove cups from pan; place on individual serving plates. Sprinkle with olives, tomato and onions.