Carefully slice the oranges into ¼-inch thick rounds using a sharp knife or a mandoline.
Combine the sugar and water in a saucepan and warm it up, stirring until the sugar fully dissolves and the mixture starts to boil.
Lower the heat to maintain a gentle simmer, then lay the orange slices in a single layer inside the syrup. Let them cook for 45 to 60 minutes, turning them now and then until the peels become nearly see-through.
Using tongs, remove the oranges and place them on a wire rack. Leave them out uncovered at room temperature for 24 to 48 hours until they feel sticky but firm.
For an extra touch, optionally roll the slices in sugar for a frosted look or coat them with melted chocolate and sprinkle with flaky salt before serving or storing.
