Sautè the minced onion, carrot, and celery in butter until fragrant and soft, approximately 5 minutes, then set aside.
Preheat the oven to 350º. Add cubed bread to an oven-safe baking dish.
To the bread cubes, add then whisked egg, and spoon the mirepoix over the top then toss to combine and distribute the sautéed vegetables.
Pour the vegetable stock into the baking dish, coating all of the bread to make sure the bread gets wet with liquid.
Cover with aluminum foil, then bake for 30 minutes. Uncover, then bake for an additional 15 minutes.
Notes
Be sure to coat ALL of the cubed bread when adding the liquid as to ensure none of it dries out as it bakes. In my testing, I determined that 3 cups of liquid is plenty to get that jiggly, soggy, texture a classic stuffing is known for!
If you are going to make your own stuffing cubes, make sure the bread is stale so that it absorbs all of the liquid as it bakes.