Preheat oven to 400°F. Spread cornbread cubes in an even layer on a baking sheet and toast until crisp, 15 – 20 minutes.*
Heat an oven-safe/burner-safe dish with a lid over medium heat. Add bacon and cook to render the fat, 5 – 7 minutes. Remove the bacon to a paper-towel lined plate to drain and pour off all but 1 Tbsp of rendered bacon fat.
Add onions and celery to pot with reserved 1 Tbsp of bacon fat. Saute over medium heat until soft, 3 – 5 minutes. Add sage and garlic and cook until fragrant, about 1 minute.
Add broth and reserved bacon and mix to combine. Add cornbread cubes, gently mix and remove from heat. Cover and let rest for 10 minutes.
Remove lid and transfer dish to the preheated oven. Bake until top is browned and crisp, about 30 minutes.