Preheat oven to 350F degrees.
Make a hot water bath for flan by setting a 9-inch pie pan (1-1/4 " deep) in a slightly larger pan. While holding pie pan down so it won't float, fill outer pan with just enough hot water to come up around pie pan. Remove pie pan and put outer pan with water in preheated oven.
Place 3/4 cup of the sugar in a small heavy pan over medium heat. As sugar begins to melt, tip and swirl pan until sugar is completely melted and syrup is a clear, medium-amber color. Pour syrup into pie pan and, using hot pads to protect hands, tilt pan quickly to let syrup coat bottom and sides. Set on a wire rack; caramel will harden rapidly.
In a large bowl or blender, combine remaining 1/4 cup sugar, orange juice, lemon juice, orange peel, and eggs; beat or whirl until well blended. Set caramel-lined pan in hot water bath in oven; pour in egg mixture. Bake, uncovered, for about 25 minutes or until the back of a spoon pushed gently into center forms a crevice about 3/8 inch deep.
Remove pie pan from hot water and let cool slightly; then cover and refrigerate until cold. As flan cools, caramel liquefies.
To unmold, loosen custard edge with a knife, then cover with a rimmed serving plate. Holding plate in place, quickly invert, turning out custard with caramel on top. Garnish with orange slices.