In a 3- to 4-quart pan, mix sugar and cornstarch. Add mango nectar and stir until blended. Stir over high heat until mixture boils, about 3 minutes. Remove from heat and stir in lemon peel and juice. Add mangoes and mix gently to coat. Pour into a shallow 2 1/2- to 3-quart baking dish.
In a bowl, mix oats, flour, nuts, ginger, and salt. Add syrup and butter and stir until blended. Distribute mixture evenly over mangoes. Set dish in a foil-lined 10- by 15-inch baking pan.
Bake in a 375° oven until topping is golden brown, 30 to 40 minutes. Serve warm or cool.