Preheat the oven to 300F. Place the cubed bread on a rimmed baking sheet and drizzle with the olive oil. Add a pinch of salt and toss to evenly combine. Bake, stirring every 10 minutes, for 30-35 minutes until dry and slightly toasted. Let cool.
In a skillet over medium-low heat, add the butter and melt. Add the onion,celery, and ¼ teaspoon salt and cook until completely soft, about 15 minutes. Add the garlic, thyme, sage, pepper, ¼ teaspoon salt, and gluten-free soy sauce (if using) and cook for another 1-2 minutes. Remove from heat and set aside.
**To make-ahead stop here and store the celery/onion mixture in the fridge until the next day and keep the toasted bread at room temperature.
Preheat the oven to 375F. Heat the chicken stock in the microwave or in a skillet until just simmering. Place the eggs in a bowl and whisk to combine. Slowly add the hot stock to the eggs, starting with just a few tablespoons while whisking constantly. Add the rest of the stock while whisking.
Place the bread cubes, celery/onion mixture and stock/egg mixture in a large bowl and gently stir to combine. Pour into a greased 9x13 baking dish and cover tightly with tin foil.
Bake covered for 30 minutes. Uncover and continue to bake for another 10-15 minutes until golden and toasted on top. Serve and enjoy!