Preheat the oven to 450°. Butter a 13-by-9-inch baking pan. Line the bottom of the pan with wax paper; butter the paper. Dampen a clean kitchen towel and sprinkle it generously with sugar. Using a sharp knife, peel the oranges, removing all of the bitter white pith. Thinly slice the oranges crosswise.
In a large bowl, combine the eggs with 1 cup of the sugar, the orange juice and zest. Using a hand-held mixer, beat on high speed until the mixture is pale and thick, about 1 minute. Scrape the mixture into the prepared pan and bake for about 5 minutes, or until puffed and browned on top.
Remove the dessert from the oven and let stand for 2 minutes. Run a sharp knife around the edge and invert it onto the sugared towel. Peel off the wax paper. Lift half of the towel up to fold the dessert in half. Cut it crosswise into 8 slices, garnish with the oranges and mint leaves and serve.