At least 24 hours in advance, cut a loaf of gluten free bread into cubes and spread onto a baking sheet. Leave bread cubes out to dry until you're ready to prepare the stuffing.
The day of - Heat the olive oil in a skillet over medium heat.
Add the onions and celery to the skillet and cook for about 5 minutes, stirring occasionally.
Add the seasoning and garlic to the onion mixture. Cook an additional 3 minutes.
Place your bread cubes into a large bowl and pour the onion mixture on top.
In a small bowl, combine the egg, chicken broth and melted butter. Whisk together. Pour over the bread cubes.
Stir the mixture gently until all of the cubes are coated in the seasoning. Cover the bowl with plastic wrap and place in to refrigerator for 30 minutes to soak.
Preheat the oven to 350 degrees. Spray a 9x7 casserole dish (or an 8x8 dish) with nonstick spray.
Pour the bread cube mixture into your casserole dish and cover with aluminum foil. Bake in preheated oven for 35 minutes. Then remove the foil and bake an additional 20 minutes until the top is golden brown and crispy.