The night before, cut or tear the loaf into chunks and spread the pieces out on a baking sheet, leaving them to sit overnight or until they are dried out. You can also dry the bread out by placing the baking sheet in a 170-degree oven for 15-20 minutes.
Preheat your oven to 375 degrees. Grease a medium-size baking dish and set aside.
In a large pot, melt butter and add in leeks, celery hearts, onion, and thyme. Heat until the vegetables are fragrant and softened. Pour the bread into the pot and toss to coat evenly; pour the mixture into a large bowl and toss in the egg, coating as evenly as possible.
Add broth; you may need more than 1/2 cup. The goal is to hydrate the bread but not leave it sitting in liquid. Fold in the chopped olives, sprinkle in the umami powder, season with salt and pepper, and pour into the prepared baking dish. Bake for 30 minutes or until the top is browned, but making sure the center is still moist.