Spread toasted bread slices on baking sheets and let sit out at room temperature until hard, about 24 hours.
Once bread slices are ready, gather all ingredients.
Preheat the oven to 325 degrees F (165 degrees C). Lightly grease a 9x13-inch baking dish.
Crush bread slices into crumbs with a rolling pin. Transfer crumbs to a large bowl.
Melt butter in a medium saucepan over medium heat. Stir in onion and celery; cook and stir until soft, 5 to 7 minutes. Remove from heat and drain.
Add chicken broth and eggs to bread crumbs; mix until combined and moist, but not mushy. Add water, if necessary, to attain desired consistency. Mix in cooked onion and celery, sage, garlic powder; season with salt and pepper.
Press dressing mixture into the prepared baking dish.
Bake in the preheated oven until the top is brown and crisp, about 1 hour.