Preheat oven to 350°F. Lightly grease a 9×13-inch baking dish and line two baking sheets with parchment paper.
Cut sourdough bread into 1-inch cubes and spread it evenly on lined baking sheets. Bake for 20-25 minutes or until bread is completely dry and starts to crisp up and get golden brown. Remove from oven and transfer to a large bowl to cool.
1 loaf sourdough
While bread is in the oven, heat butter in a large skillet over medium heat. Add onion, garlic and celery and sauté until onion is fragrant and translucent, about 5-7 minutes.