Preheat oven to 375°. In a large skillet, cook beef over medium heat until no longer pink; drain and set aside. In the same skillet, heat oil over medium-high heat. Add zucchini, onion and green pepper; cook and stir until crisp-tender, 4-5 minutes. Stir in beef, tomato puree, oregano, salt and pepper; heat through.
Spread mashed potatoes in an 11x7-in. baking dish coated with cooking spray. Top with beef mixture; sprinkle with feta cheese.
Unroll crescent dough. Separate into 4 rectangles; arrange 3 rectangles over the casserole. If desired, brush with egg wash. Bake until top is browned, 12-15 minutes. Roll remaining dough into 2 crescent rolls; bake for another use.