Preheat oven to 350° F. Defrost spinach according to package directions. Rinse and set aside.

Place sourdough cubes onto baking sheet and bake 8-10 minutes. Flip cubes and bake for 5 minutes longer or until edges of cubes are crunchy. Set aside. Maintain oven heat.

In large pan, heat butter over medium-high heat. Add onions and celery and cook until they begin to soften, about 3 minutes. Add garlic. Cook 2-3 minutes longer.

Add in broth and bring mixture to a boil. Reduce heat to medium-low and stir in bread cubes. Cook until mixture has softened the bread and the broth has been absorbed, about 5 minutes more. Remove from heat. Stir in spinach and half of mozzarella and half of Parmesan until mixture is fully combined.

Add mixture to lightly greased 9x13-inch baking pan. Top with remaining mozzarella and Parmesan cheese. Bake in preheated oven for 25 minutes or until cheese is melted and bubbly.