In a food processor, combine brown rice flour, all-purpose flour, cornstarch, coconut sugar, and sea salt. Pulse cold, diced butter into the mixture until it resembles coarse crumbs, about 20 seconds.

Gradually add ice water, pulsing until the dough just holds together. Turn the dough onto a floured surface, knead gently, shape into a disk, wrap in plastic, and chill in the refrigerator for at least 30 minutes.

While the dough chills, prepare the filling. Cut fresh rhubarb into 1-inch pieces, and in a mixing bowl, gently stir together with honey, dried lavender, vanilla essence, lemon juice, and raspberries. Let the mixture sit for about 10-15 minutes.

Preheat your oven to 400°F (200°C). Roll the chilled dough on a floured surface into a rough 12-inch circle, then transfer it carefully to a parchment-lined baking sheet.

Spoon the fruit filling in the center of the dough, leaving a generous 3-inch border. Fold the edges of the dough over the filling, pinching and pleating to create a rustic appearance.

Chill the assembled galette in the fridge for about 15 minutes. Brush the edges with egg wash and sprinkle slivered almonds and coarse sparkling sugar on top. Bake for 45-50 minutes, until the crust is a deep golden brown.

Once baked, allow the galette to cool on the baking sheet for at least 10 minutes. Slice and serve warm, ideally with a scoop of vanilla ice cream.