Preheat oven to 400 degrees F. In food processer fitted with S-shaped blade, combine cassava flour, almond flour, arrowroot, salt and thyme. Add butter and pulse until it forms pea-size pieces.

Transfer mixture to bowl and set aside. In small bowl, combine water and egg yolk. Pour into flour mixture, stirring with fork until fully incorporated. Form dough into 6-in. disc, wrap and refrigerate 30 minutes.

To prepare filling, warm medium sauté pan over medium heat. Add oil, onion and garlic and sauté 3-4 minutes until onions soften.

Add mushrooms, salt and thyme and sauté 2 minutes. Add Brussels sprouts and sauté 2-3 minutes more.

Transfer filling to bowl, stir in baby tomatoes and set filling aside to cool slightly.

Roll out crust between two layers of parchment paper. Sprinkle paper with a little cassava flour if dough is too sticky. Roll crust into a 12-in. round, (don’t worry if it’s not exact). Peel off top layer of parchment paper.

Spoon filling onto middle of crust, leaving 2-in. rim around it. Fold crust over edge of filling, toward the middle, using hands to gently press together. Sprinkle crumbled feta cheese over center.

In small bowl, mix egg and water and brush over crust. Slide galette with bottom piece of parchment paper onto sheet pan.

Bake 35-40 minutes until bottom and top crusts are lightly browned. Serve warm or at room temperature.