In the bowl of a food processor combine the brown rice flour, all-purpose flour, cornstarch, coconut sugar, and salt. Pulse once or twice to combine. Add the diced butter and pulse until you end up with small pieces of butter, similar to the size of rice. Add 3 tablespoons ice water to the dough and pulse until it comes together. If needed add 1-2 tablespoons more ice water until the dough comes together. Turn the dough onto a floured work surface, knead a couple of times and then form the dough into a flat disk. Wrap in plastic wrap and refrigerate for at least 30 minutes.
Meanwhile, make the filling. In a bowl, combine the rhubarb, honey, 1 tablespoon lavender, vanilla + vanilla bean seeds (if using), lemon juice and 1 cup raspberries. Toss well to combine.
Now grab your dough from the fridge. Flour your work surface again and roll the dough to about 1/8-inch thick. Transfer to a baking sheet lined with parchment paper. Add the filling to the center of the dough, leaving a 3-inch border around the edges. Fold the edge of the dough over the filling. Brush the crust with the beaten egg and scatter the slivered almonds over the dough.
In a bowl, combine the remaining 1 tablespoon dried lavender with 1/2 cup sugar. Sprinkle the galette with the 1-2 tablespoons of the lavender sugar (save the rest for serving and/or another use). Place the galette in the fridge for 15 minutes or until ready to bake.
Preheat the oven to 400 degrees F. Remove the galette from the fridge and add the remaining raspberries and butter right on top. Bake the galette for 45-50 minutes or until the crust is golden and the fruit soft and caramelized. Allow to cool completely before slicing.
Serve slightly warm or at room temp with a large scoop of vanilla ice cream, lavender sugar and/or a drizzle of honey.
