• In a saucepan, combine 1 cup sugar and ½ cup water over medium heat. Stir until sugar dissolves.
• Add orange slices and simmer for 15 minutes, until the rinds become slightly translucent. Set aside in the syrup.
• Toast fennel seeds in a dry skillet over medium-high heat for 3 to 4 minutes until fragrant.
• Reserve ¼ teaspoon of seeds and grind the remaining seeds into a fine powder.
• Preheat oven to 350°F. Grease and line an 8-inch round cake pan and place on a baking tray.
• Scatter the reserved fennel seeds in the bottom of the pan. Arrange orange slices in a spiral pattern.
• Drizzle 3 tablespoons of the orange syrup over the oranges and reserve the remaining syrup.

Make the Cake Batter

• Beat butter, remaining sugar, and orange zest until pale and creamy.
• Add eggs one at a time, mixing well after each addition.
• Fold in flour, baking powder, ground fennel seeds, and salt.
• Fold in the Greek yogurt until combined.
• Spoon batter over the oranges and smooth the surface.

Bake

• Bake for 50–55 minutes, until golden and springy in the center.
• Check after 40 minutes and cover with foil if browning too quickly.
• Cool for 5 minutes, then run a knife around the edge and invert onto a serving plate.
• Serve warm with crème fraîche and a drizzle of the reserved orange syrup, or allow the cake to cool completely.