Preheat the oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil. Poke holes all over the surface of the butternut squash using a fork. Place on the baking sheet.
Bake squash in the preheated oven until tender, about 50 minutes. Remove and let cool. Reduce oven temperature to 365 degrees F (185 degrees C).
Combine flour, mozzarella cheese, baking soda, baking powder, and coriander in a bowl.
Peel and cube butternut squash. Measure 1/2 cup into a blender; refrigerate the rest. Add egg, Greek yogurt, applesauce, and oil to the blender. Blend until bubbly, about 5 minutes. Pour into the bowl with the flour mixture and mix well. Spoon the thick batter into a 7 1/2-inch cast iron skillet. Sprinkle sunflower seeds on top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 40 minutes. Let cool for 5 minutes before slicing.