Cut the orange on 3 mm (0.1 inches) thick slices. Make sure you are using bio oranges that have an edible peel. Place a medium saucepan over high heat. Add water and sugar, stir to combine and let the sugar dissolve. You need to get a nice, translucent sugar syrup. Add the orange slices. The syrup should cover the oranges.
Bring to a boil, then lower the heat. Cover with a lid and cook for about 45 minutes (low simmer). Remove from the heat and let the orange slices cool completely while still in the saucepan, covered with the syrup.
You can use candied orange slices in three ways. Way number 1: Store the candied orange slices covered with syrup in an airtight container in a fridge for up to two months. They are great for decorating cakes. Way number 2: Drain the candied orange slices, then transfer to a wire rack for the orange slices to dry. Leave them to dry for about 24 hours at room temperature. This is a great way to store them later in a pantry. Store in an airtight container for 2 - 3 months or use them in pudding, mousse or panettone. Way number 3: Drain the candied orange slices, then transfer to a wire rack for the orange slices to dry. Leave them to dry for about 48 hours at room temperature, then cover them in white sugar. Store in an airtight container for up to 3 - 4 months. Great as a gift for Christmas.