Pre-heat your oven to 400 degrees.
Once your dough is cooled, transfer to a surface sprinkled with flour. Remove the dough from the plastic wrap and add about another 1/4 of flour to the surface of the dough. Smush the dough ball into a flat circle and roll with a rolling pin until about 1/4 of an inch thick, and about 12 inches in diameter.
In a separate bowl, stir together goat cheese and mushroom mixture until fully combined (make sure to remove thyme sprig prior). Pour it into the middle of your galette. I like to place a few of the cooked mushrooms in the middle so they are visible. Leave about 1 1/2 inches on the sides of dough.
Place the thin cut circles of watermelon radish around the outside of the filling.
Now fold your galette dough over the filling, pleating as you go (see video). Leave an opening in the middle.
Whisk 1 egg in a small dish and brush over the the pleated crust.
Place in the oven on higher rack and bake until crust turns golden, about 45 minutes. Take out, let cool a few minutes before transferring to a serving dish. Drizzle with agave or honey and enjoy!