You will need a stand mixer fitted with a whisk attachment, and a small offset spatula.
To prepare the orange slices:
- Prepare a carouche – a paper lid to keep the oranges submerged in the syrup while they are poaching. Do this by cutting a circle out of parchment paper sized to just fit inside a small saucepan. Make a small hole in the centre for venting.
- Slice the orange as thinly as possible while keeping the fruit intact.
- In a small saucepan, combine the sugar, water, vanilla bean, and spices. Bring to a boil.
- Reduce the heat to minimum and lay the orange slices in the syrup so that they overlap and form a circle. Fit the cartouche snugly overtop. Simmer on low for two hours, or until the orange slices are soft, have absorbed some of the syrup, and look slightly translucent. To keep the delicate orange slices from falling apart, make sure the simmering temperature is as low as possible and your syrup does not boil.
- Remove from heat and let cool. Cover the saucepan and refrigerate overnight.
For the frangipane:
- Preheat your oven to 375F.
- In a bowl, mix together the almond flour, all-purpose flour, orange zest and hazelnuts. Set aside.
- Put the egg and sugar in a stand mixer bowl and whisk on medium speed until frothy. Slowly stream in the melted butter and mix until well incorporated.
- Remove the bowl from the mixer and gently fold in the dry ingredients and the sour cream until just combined.
- Using a small offset spatula, gently spread the frangipane batter over the bottom of the tart, taking care not to break the shell. The batter should fill up the tart about halfway. Be careful not to overfill, as the frangipane will rise in the oven. Discard any leftover frangipane.
- Bake for 18 to 20 minutes, until the frangipane is golden brown and has puffed up. Prepare the caramel glaze while the frangipane is in the oven.
For the caramel glaze:
- Make sure the water is ice cold. Put it in a small microwaveable bowl, sprinkle in the gelatin, and stir to dissolve. Set aside at room temperature until firmly set, about three to four minutes.
- Heat the cream in a microwave or a small saucepan on the stove until scalding. Set aside as you melt the sugar.
- Place about a quarter of the sugar in a saucepan over medium heat. Gently melt the sugar, swirling the saucepan around as needed. Do not stir. Once the sugar is almost melted, sprinkle another quarter of the sugar into the saucepan and continue to swirl. Repeat twice more and cook until all the sugar is completely melted and has turned amber in colour.
- Remove from heat and slowly pour in the hot cream. Be sure to pour carefully as the mixture will bubble up. Using a heatproof spatula or a wooden spoon, mix the caramel until smooth. Using a fine mesh strainer, immediately strain the caramel into a heatproof bowl.
- Mix the butter into the caramel until it is fully melted and well incorporated.
- Melt the set gelatin in the microwave for 1 minute, or until completely liquid, and then stir it into the caramel. Set aside and let cool slightly.
To finish the tart:
- Brush the baked frangipane generously with orange poaching syrup.
- Pour on a layer of caramel. Refrigerate until set, about 20 minutes.
- When the caramel is set, gently remove the orange slices from the syrup. They will be quite soft so be careful not to tear them. Using a paper towel, pat the excess syrup off the slices and arrange them over the caramel. Sprinkle with chopped hazelnuts to finish. Serve at room temperature.
Storage: This tart will keep at room temperature for up to three days.