Place first 5 ingredients in a pot over medium-high heat and bring to a simmer, stirring to dissolve the sugar. Simmer 5 minutes, then carefully place the orange slices in the syrup. Simmer 1 minute more, then remove from heat and cool to room temperature. Place in the fridge until well chilled. Divide and arrange the orange slices on 4 plates, then top with some of the syrup. Serve the oranges with a small spoon of vanilla ice cream, whipped cream or mascarpone cheese alongside. Garnish with mint sprigs.