Preheat the oven to 350ºF (180ºC). In a non-stick ovenproof 9 inch skillet, melt the caster sugar and water. Cook until golden. Add orange slices and cook until soft and caramelized, about 15 minutes. (Add one teaspoon of water if caramel is getting too dry before the oranges are done.) Remove from the heat, arrange to desired pattern, and set aside in the pan. Let the orange slices and caramel set up a bit to ensure you get a nice pattern on your final cake.
In a medium bowl, gently whisk together flour, ground almonds, baking powder and salt; set aside.
In a measuring cup, combine the blood orange juice, zest, Cointreau, and yogurt. In a mixer fit with a whisk attachment, beat sugar and eggs together until thick, foamy, and tripled in volume, about 8-10 minutes. Gently stir in liquids into the egg mixture. Gently fold the flour mixture in until just combined.
Pour cake batter into the skillet over the caramelized orange slices and bake 20 to 30 minutes. Remove from oven and let stand for 5 minutes before inverting onto cooling rack. Allow to cool completely.
For the caramelized orange slices
½ cup caster sugar
¼ cup water
2 blood oranges, very thinly sliced
For the cake
½ cup (100g) granulated sugar
2 large eggs, room temperature
¾ cup (80g) ground almonds
¾ cup (100g) all purpose flour
1 tsp baking powder
Pinch of salt
1 blood orange, juice and zest (about 100ml juice)
2 tbsp plain yogurt (optional)
1 tbsp Cointreau (orange liqueur)