1. In a separate bowl, combine dough ingredients until well mixed. You should have a soft dough.
2. Preheat oven to 350°F (180°C). Divide dough in half; press half of dough into a 9×13-inch baking pan to form a thin crust. (You do not need to go up the sides of the pan.) If the dough is too thick, pull off excess and add to reserved half.
3. Spread rhubarb-berry filling over crust. With floured hands, roll remaining dough into long, thin strands. Use additional flour as needed. Place half the ropes diagonally across pan, and remaining ropes across the other way, forming a lattice design. Brush ropes with reserved white, if desired. Sprinkle with coarse sugar and bake for 45 minutes to 1 hour, until filling is bubbly and crust is golden brown.