Follow the instructions for my gluten-free pie crust, whisking in the rosemary with the flour. Shape into a small rectangle and refrigerate for 1 hour.
Preheat oven to 350 degrees. Remove dough and roll between 2 pieces of parchment paper. Transfer to your tart pan and trim the edges. Gently press the dough into the tart pan. Blind bake in the oven for 15 to minutes until edges are firm, but not browned. Cool on a wire rack.
In a small saucepan, whisk together the lemon juice, zest, maple syrup, eggs, arrowroot, and salt. Heat over medium-low, stirring frequently until thick, about 10 minutes. Remove from heat and stir in the dairy-free butter.
Pour the mixture through a sieve to remove any curdles lumps. Pour filling into baked pie shell and arrange lemons slices on top. Bake for 15 minutes, until set. Cool completely.