Line a jelly-roll pan with wax paper. Cut citrus into ⅛-inch-thick rounds; discard seeds. In a large skillet, heat sugar, ¾ cup water, and lemon juice over medium heat until sugar is dissolved. Add citrus slices, and simmer gently, keeping slices in a single layer, turning occasionally, until slightly translucent and rinds are softened, 14 to 16 minutes. Remove from heat. Using tongs, place slices in a single layer in prepared pan. Let cool completely, about 1 hour. Cover and refrigerate for at least 2 hours or up to 2 days. Sprinkle with additional sugar just before placing on cake. Reserve syrup for another use, if desired.
Recipe by Bake from Scratch at https://bakefromscratch.com/candied-citrus-cake/