Begin by washing the orange carefully under running water, then slice it thinly, keeping each slice about ΒΌ inch or 0.5 cm thick.
Pour the water and sugar into a wide-bottomed pot, heating gently until the sugar dissolves completely. Add the orange slices in a single layer as much as possible, letting them overlap only slightly since they will shrink during cooking.
Simmer the slices on low heat for 40 minutes, turning them now and then for even cooking. Afterwards, place the slices on a wire rack to cool fully; you can refrigerate them for 10 to 15 minutes or cool in the fridge for 10 minutes plus the freezer for 5 minutes to speed this up.
Melt the dark chocolate over simmering water using a double boiler technique. Dip each slice halfway into the melted chocolate, then set them on a tray lined with foil.
Optionally, sprinkle coarse salt over the chocolate before the coating sets.
Chill the dipped slices in the refrigerator or store in a cool spot until the chocolate hardens completely, which should take about an hour.
Serve and enjoy this simple yet elegant treat perfect for festive occasions or as a sweet snack.