In a large skillet, cook beef over medium heat until no longer pink; drain and set aside. In the same skillet, saute the zucchini, onion and green pepper in oil until crisp-tender. Stir in the beef, tomato puree, oregano, salt and pepper; heat through.
Spread mashed potatoes in an 11-in. x 7-in. baking dish coated with cooking spray. Top with beef mixture; sprinkle with feta cheese.
Unroll crescent dough. Separate into four rectangles; arrange three rectangles over the casserole. Bake at 375° for 12-15 minutes or until top is browned. Roll remaining dough into two crescent rolls; bake for another use. Yield: 6 servings.