Preheat oven to 350 degrees. In a medium bowl, mix together almond flour, rosemary, salt and baking soda. In a small bowl, mix together olive oil and water. Add olive oil and water mixture to dry ingredients and stir until combined. Press into 9" tart pan. Bake 15 minutes until golden brown; remove from oven and let cool.
Slice tomatoes into 1/2" slices; lightly salt both sides of tomatoes; place on double paper towel lined baking sheet; let sit 10 minutes; flip tomatoes over and let sit another 10 minutes.
Lay tomatoes in baked pie crust and put onion slices on top; sprinkle with chopped basil and season with garlic powder and ground pepper.
Mix mayonnaise and 1/2 cup Parmesan cheese together in a small bowl. Gently spread on top of tomato pie. Sprinkle remaining 2 tablespoons Parmesan cheese on top.
Bake 15 minutes until top is golden brown.
Pie crust adapted from The Gluten-Free Almond Flour Cookbook by Elana Amsterdam.