Preheat oven to 350 degrees. Make orange-flower syrup: 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.
Make candied hazelnuts: Place nuts
in a small bowl. Whisk egg white in another
small bowl until foamy. Stir 2 to 3
tablespoons whipped egg white into the
nuts (enough to coat nuts). Add brown
sugar and cinnamon; stir to coat nuts.
Transfer hazelnuts to a rimmed baking
sheet. Bake until nuts are dry, 5 to 7 minutes.
Cut oranges into 1/4-inch-thick rounds. Divide among serving dishes. Drizzle orange-flower syrup on top; sprinkle with candied hazelnuts, dividing evenly. Garnish with rosemary sprigs, if desired.
Let cool completely.