Preheat your oven to 350 degrees F (175 degrees C).

Place the peeled and cubed potatoes in a pot of salted water and bring to a boil. Cook until tender, about 15 minutes.

In a skillet, melt the butter and sauté the onion and celery until translucent, about 5-7 minutes. Add garlic and cook for another minute.

In a large bowl, combine the bread cubes, lightly mashed potatoes, sautéed onion and celery, parsley, thyme, salt, and pepper. Mix gently.

Whisk together the chicken broth and beaten egg. Pour over the bread and potato mixture and toss gently to combine.

Transfer the mixture to a greased baking dish, spreading it evenly.

Bake for approximately 30 minutes until the top is golden brown and crispy.