Make the crust by placing the flour, salt, and cold butter into a food processor and pulse 7-8 times until the mixture looks like a loose crumb. Stream in a 1/4 cup water until the dough just comes together. It should be crumbly but when you squeeze a handful of it together, it should stay in a clump. If it does not clump, add the remaining ice water, pulsing a few more times.

Turn the dough out onto a lightly floured surface and mound it into a single ball. If it's still crumbly, add another tablespoon of water until everything holds together. Do not overwork the dough.

Once it all holds together, split the mound in half. Form each half into a round, flat disc. Wrap each in plastic wrap and place in the fridge for 30-45minutes to chill.

While the dough chills, place the cut rhubarb in a medium-size bowl and cover with cool water. Stir in a 1/4 cup sugar and allow the rhubarb to soak for at least 30-minutes. After 30-minutes, drain the water from the rhubarb, dry off the pieces on a piece of paper towel, then place the rhubarb pieces into a clean, dry medium-size bowl. Add in the raspberries, the remaining 1/4 cup of sugar, lemon juice, and tapioca flour and stir to combine. Allow the fruit to sit while you prep the pastry.

Preheat the oven 400F and line a baking sheet with parchment or a silicone baking mat.

Remove the pastry discs from the fridge. Place a single disc onto a lightly floured surface and roll it into a 9-inch circle. Gently move the rolled circle to the prepared baking sheet and repeat with the remaining disc.

Pile the fruit mixture into the center of each pastry circle, mounding into the middle with about 1-2 inches border. Gently fold the excess pastry up around the fruit, crimping the edges so they stay in place.

Using a pastry brush, gently brush the pastry with 2 tablespoons heavy whipping cream and sprinkle with sugar. Bake for 30-35 minutes in the oven until the pastry is golden brown. Allow to cool for at least 10-minutes before serving.

While the galettes are cooling, make the whipped cream by placing the 1/4 cup of heavy whipping cream in a small bowl with 2 tablespoons of confectioner's sugar. Using a handheld mixer, mix the heavy whipping cream until thick and fluffy.

Serve galettes with a dollop of whipped cream on top.