Mix together the almond flour, sugar, and salt together. Cut in the butter until the mixture is mealy. Mix in the egg until well combined and a crumbly dough forms. It should stick together if you squeeze it in your hands. Equally divide the dough by four and press into tart molds. Prick the bottoms of the tarts with a fork. Chill the molds in the refrigerator for at least 30 minutes, or overnight.

Preheat the oven to 400 degrees and bake for 10 - 14 minutes, until golden brown. Set aside to cool.

In a small saucepan, boil the grapefruit juice until only about ½ a cup of juice remains. Boiling concentrates the flavor for the best results.

In a double boiler, whisk together the sugar, salt, zest, ginger juice, grapefruit juice, eggs, egg yolks, and vanilla bean together. Cook on the double boiler, whisking constantly, until the mixture becomes very thick. This can take anywhere between 10 - 15 minutes. Add in the butter and whisk until the butter is completely melted.

Strain the mixture through a fine mesh strainer. Allow the curd to reach room temperature before putting it in the refrigerator to finish setting. The mixture will continue to thicken as it cools.

In a small saucepan, place the sugar, water, ginger, and cardamom together. Bring it to a boil and then simmer for about 3 - 4 minutes until thick and syrupy. Set aside to cool.

In a freshly cleaned and dried mixer, whip the egg whites until soft peaks form. Add the cream of tartar and whip for another 30 seconds. Slowly drizzle in the syrup and whip until stiff, glossy peaks form.

Fill the tarts with the grapefruit curd. You should have exactly enough curd to fill each tart almost completely.

Place the meringue in a piping bag and pipe a large amount of meringue over the curd, making sure to cover the curd entirely. Alternatively, you can use two spoons to scoop the meringue over the curd. I find this creates a more rustic look.

Use a kitchen torch or oven broiler to torch the meringue and lightly brown.