Combine potatoes & 5 c. Water in a large saucepan; bring to a boil. Cover, reduce heat & simmer 15 min. Or till tender. Drain potatoes & chill. Cut potatoes into 3/4 in. Cubes. Combine potatoes, cucumber, & tomatoes in a large bowl. Set aside. Drop garlic through chute in processor with processor running. Process about 3 sec. Or until garlic is finely chopped. Add yogurt &feta cheese, oregano & rosemary. Process until smooth. Add yogurt mixture to reserved potato mixture; toss gently to coat. Garnish with parsley, olives & onion rings if desired. Cover & chill thoroughly.