To make the filling: Toss the berries and arils together gently. Add the sugar and the lemon juice and toss to combine. Add the tapioca starch and combine gently. Set filling aside while you roll out the dough.
Line a cookie sheet with parchment paper. Roll out the dough to a large circle (at least 10" diameter). Transfer the rolled out dough onto the pan.
Place the filling in the center of the circle leaving a 3" border. Fold the dough over, toward the center. Chill in the freezer.
Preheat the oven to 400 F. Brush the edges of the crust with eggwash and sprinkle with sugar. Add 1 tablespoon of butter to the top of the galette, right in the center (so it melts onto the filling). Bake for 25-30 minutes until the edges are golden.