Cut a slice off the top and bottom of each orange to expose the flesh. Cutting from top to bottom, following the contours of the orange, completely remove the peel and white pith. Discard the peel and set the oranges aside. In a saucepan, combine all of the remaining ingredients except the mint leaves. Heat to a simmer. Add the whole peeled oranges and simmer, uncovered, 10 minutes, turning the oranges every 2 minutes. Remove from the heat and set aside to cool to room temperature in the liquid, turning occasionally. Chill. To serve, cut each orange crosswise into 1/2-inch-thick slices. Spread the slices, overlapping them, on small serving plates. (The outsides of the oranges will be redder than the insides.) Drizzle a little liquid over each plate and sprinkle with mint.