I made this recipe using blood oranges, which are a variety of orange with crimson, almost blood-colored flesh, but any oranges would work equally well for this tart. It tastes as good as it looks!
In a saucepan, whisk egg yolks and sugar until smooth and combined.
Zest and juice the oranges and add to the pan.
Turn on the heat and cook slowly, stirring until the curd is thick enough to coat the back of a spoon.
Remove the saucepan from the heat and whisk in butter or margarine, one cube at a time, until the curd is thick and smooth. Leave to cool in the fridge.
Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).
In a bowl, mix ground hazelnuts, sugar, matzo meal, potato starch, vanilla extract, egg whites, and salt until well combined.
Spoon the mixture into a nine-inch (23-centimeter) tart pan with a removeable bottom. Press into the bottom and up the sides. Cover with aluminum foil and add baking beans.
Blind bake for 25 minutes or until golden.
Remove from the oven and leave to cool. Carefully remove from the pan and transfer to a serving plate.
Place orange slices in a frying pan in a single layer. Line a plate with parchment paper and set aside.
Add sugar, water, cinnamon, and mixed spice (if using). Simmer 10 minutes or until the sugar starts to caramelize.
Remove and leave to cool on the prepared plate.
Spoon the curd into the cooled tart base and spread in an even layer.
Decorate with caramelized oranges, chopped hazelnuts, and orange zest.
For the best flavor, roast whole hazelnuts and grind in a food processor. (Roast an extra three tablespoons hazelnuts to reserve for the topping.)
Don’t have baking beans? You can also use granulated sugar. Simply line the tart base with foil and pour sugar on top to cover. After blind baking, you can reuse the sugar in another recipe.