1 cup water
1 cup sugar
1 blood orange
1. Bring water and sugar to boil in a heavy large skillet, stirring until the sugar dissolves.
2. Thinly slice blood orange (you can substitute a regular orange); add to skillet, arranging in a single layer. Reduce the heat to medium-low and barely simmer until the white pith of the orange becomes translucent, turning the slices occasionally, about 40 minutes.
3. Allow the orange slices to cool in the syrup, turning occasionally. Arrange the slices atop the tart and drizzle with some of the syrup just before serving.