2. Crust, combine flour and sugar. Cut in butter. Press into 13x9x2 pan. Bake for 7-10 minutes or until lightly brown.
3. Filling, combine in a saucepan mangoes, sugar, water and lemon juice. Cook over low-medium heat until mangoes are tender, 10-12 minutes. Combine cornstarch and water in a separate bowl. Stir into mango mixture. Cook until mixture is thick. Cool slightly. Pour over prepared crust.
4. Topping, combine oats, flour, and sugar. Cut in butter. Sprinkle over mangoes.