Preheat oven to 350 degrees. Stir together orange juice, rosemary, honey, and orange-flower water in a small saucepan. Bring to a boil over medium-high heat. Cook, skimming foam from surface with a slotted spoon, until mixture has reduced to 1/3 cup, 7 to 9 minutes. Let syrup cool completely.
Place chopped nuts in a small bowl. Whisk egg white in another bowl until foamy. Stir 2 to 3 tablespoons whipped egg white into the nuts. Add brown sugar and cinnamon; stir to coat nuts. Transfer hazelnuts to a rimmed baking sheet, and spread in an even layer. Bake until dry, 5 to 7 minutes. Let cool completely.
Slice oranges into 1/4-inch thick rounds. Divide evenly among serving dishes. Drizzle orange-flower syrup on top; sprinkle with candied hazelnuts. Garnish with rosemary sprigs.