Heat the oven to 400F. Slice the delicata squash in half lengthwise, clean out the seeds (wash and toast those as well!), and then slice the delicata into 1/4-inch thick crescent moon shapes (see photo). Toss with a generous splash of olive oil and the sliced shallots, sprinkle with a bit of salt, and arrange on a baking sheet. Bake until the squash starts to take on some color - 20 - 25 minutes. Allow to cool.
Remove the dough from the refrigerator and allow to soften for 5-10 minutes. Roll out on a lightly floured countertop (or parchment paper) until 1/4-inch thick. Transfer to a baking sheet. Spread the yogurt mixture across the galette to within an inch of the edge. Layer with the pre-roasted delicata squash and shallots, tear the mozzarella into pieces and add that as well. Finish with most of the scallions. Fold the crust edges in toward the center about an inch or so, working clockwise (see photo). Whisk the egg yolk with 2 tablespoons of water and brush the crust. Bake for 30-35 minutes or until the crust is beautifully golden.
Makes one 10-inch galette.
*If you don't have sourdough starter, you can make this tart crust instead.