Slice oranges very thinly – aiming for just slightly thinner than 1/4″. Remove and discard all seeds, if applicable
In a large pot, bring water and sugar to a boil. Add orange slices, gently stirring to separate and coat with sugar water.
Once syrup comes back to a boil, turn temperature down enough to keep it just at a good simmer – NOT a full boil – and simmer for about 45 minutes. I like to gently stir every 10-15 minutes or so, to ensure all of the slices are getting full exposure to the syrup.
As you wait for the simmering oranges, line a baking sheet with parchment paper. Spread a good amount of sugar over the parchment paper – a cup or two, enough to get a nice layer of sugar. Set up a drying/cooling rack over the sugar pan.
Once the 45 minutes are up, remove pan from heat. Use a fork to gently remove orange slices from the syrup, allowing excess syrup to drip off into the pan. Arrange drained slices on the rack, allow to drip and cool for 30 minutes or so. Once time is up, flip them and allow to sit for another 30 minutes or so.
Gently dredge orange slices in sugar from the parchment paper, making sure both sides are evenly coated. Arrange on the rack once again, allow to dry overnight
Store in an airtight container, use within a week or so.