Preheat oven to 300 degrees F.

Dice the gluten free bread and bagels into ¼-inch cubes. Spread evenly on two parchment-lined baking sheets and drizzle with garlic olive oil. Toss to coat.

Toast at 300 degrees F for 30 minutes, stirring halfway through. Remove from the oven and cool, then increase the oven temperature to 350 degrees F.

Cook the Italian sausage in a large skillet over medium heat for 10–15 minutes, breaking it into bite-size pieces. Drain on a paper towel-lined plate and set aside.

While the sausage cooks, drain the canned water chestnuts and chop the celery and leek greens. Rinse the leeks in a bowl of water to remove any dirt, then drain well.

In the same skillet, melt the dairy free butter. Add celery and leeks and cook for about 5 minutes, until softened. Add chopped green onion tops and cook for another minute. Remove from heat.

In a medium bowl, whisk together the eggs and chicken stock.

In a large mixing bowl, combine the toasted bread cubes, cooked sausage, sautéed vegetables, salt, sage, thyme, rosemary, poultry seasoning, and pepper. Pour in half of the egg mixture and stir gently to coat. Let the bread absorb the liquid, then add the remaining mixture and stir again. Most of the liquid should be absorbed.

Lightly grease a 9x13-inch baking dish (or two 8x8-inch pans) with cooking spray or butter. Spread the stuffing mixture evenly in the pan and cover with foil.

Bake for 25 minutes, then uncover and bake 5–10 minutes more, until the top is crispy and golden brown. Garnish with fresh parsley or sage and serve warm.