In a large mixing bowl, whisk together sweet rice flour, millet flour, oat flour, tapioca flour, and kosher salt until well combined. Cut in cold unsalted butter until the mixture resembles pea-sized crumbs.

Gradually add ice water, one tablespoon at a time, until the dough holds together. Shape into a disk, wrap in plastic wrap, and chill in the refrigerator for 30 minutes.

Preheat your oven to 400°F (200°C) while the dough chills.

Roll out the chilled dough to about 1/8 inch thick and cut out circles to fit your mini tart pans.

Layer chopped olives, julienned sun-dried tomatoes, fresh thyme leaves, and sprinkle Parmesan cheese in each prepared crust.

In a separate bowl, whisk together ricotta cheese, Greek yogurt, egg whites, minced garlic, and a pinch of salt until smooth.

Fill each tart with the ricotta mixture and arrange whole olives as Jack Skellington's eyes.

Bake for 20-25 minutes, until the filling is set and the crust turns light golden brown.

Let cool slightly before serving these delightful tarts warm.