Preheat your oven to 350°F (175°C). Grease a 9-inch tart pan with removable bottom.
In a medium bowl, combine almond flour, melted butter, 3 tablespoons erythritol, vanilla, and salt. Mix until it forms a dough-like consistency.
Press the crust mixture evenly into the bottom and up the sides of the tart pan. Prick the bottom with a fork and bake for 12 minutes until lightly golden. Remove and let cool slightly.
While the crust bakes, toss the sliced apples with ¼ cup erythritol, cinnamon, nutmeg, and lemon juice in a large bowl. Set aside.
To make the caramel sauce, combine heavy cream, butter, and brown sugar substitute in a small saucepan over medium heat. Stir constantly until the mixture comes to a gentle boil, then reduce heat and simmer for 5 minutes until slightly thickened. Remove from heat and stir in vanilla.
Pour half of the caramel sauce over the pre-baked crust, spreading evenly.
Arrange the apple slices in overlapping circles on top of the caramel, starting from the outside and working your way to the center.
Bake for 35-40 minutes until the apples are tender and the edges are lightly caramelized.
Remove from oven and let cool for 15 minutes. Drizzle the remaining caramel sauce over the top before serving.
Serve warm or at room temperature. This tart is amazing on its own or with a dollop of sugar-free whipped cream!