Place almond flour and salt in a bowl of a food processor and pulse. Add cold butter, ice cold water, and egg yolk. Pulse until the mixture forms a uniform dough. If too dry, add small amounts of iced water, bit by bit.
Remove the dough from the food processor, form a disc, and wrap in plastic wrap. Place in the refrigerator for about 1 hour.
In the meantime, prepare the “apple” topping.
Peel chayotes. If you’ve got prickly chayotes that are not young and soft anymore, wear some kitchen gloves to avoid pricking your hands. Slice the peeled chayotes like you would slice apples. No need to make the slices too thin.
In a large pot, whisk together water, sweetener, lemon juice, and spices. Add in chayote slices and butter, and bring to a boil over high heat. Boil for 10 to 15 minutes, stirring occasionally. Remove from heat, let sit and cool, and drain before using.
I suggest making the custard just before use. Alternatively, make sure that sweetener doesn’t sit at the bottom by stirring the custard thoroughly before use.
Using an electric mixer, whisk eggs and sweetener for 3-5 minutes until pale yellow and creamy. Add heavy cream, vanilla, and apple brandy (Calvados), and whisk thoroughly.
Assembling and baking
Preheat the oven to 390°F (200°C). Lightly grease a 10-inch tart or pie pan.
Roll the dough out to about a 12-inch circle on a lightly floured surface. Help yourself with plastic wrap placed between the dough and the roller.
Fold loosely into quarters, and center the point in the pan.
Unfold dough and press into the bottom and up the sides. Don’t stress if it falls apart – you can stick it together again. Prick with a fork all over and bake for 10 minutes until golden. Do not over bake.
Bring the oven temperature down to 360°F (180°C).
When the dough is out of the oven, arrange chayote slices in concentric circles around the dough. Pour the custard over the chayote slices.
Bake for 35 – 40 minutes. After 30 minutes of baking, top with toasted blanched and sliced almonds.
To make sure the tart is done baking, insert a toothpick into the custard. Also, if the middle is still obviously wobbly, the tart is most likely not ready.