Preheat oven to 350F.
Line a sheet pan with parchment paper. Add the cubed bread, arrange in a single layer and transfer to the oven. Cook the bread, flipping occasionally, until it is toasted and golden, but not browned, around 15 to 20 minutes depending on how fresh your bread is. Remove and set aside to let cool.
Meanwhile, preheat a sauté pan over medium-high heat. Add the olive oil and heat until shimmering. Add the sausage mixture and cook, stirring occasionally to crumble the sausage into small pieces, until golden brown, around 12 minutes.
Add 1 tablespoon of butter, if using, and heat until melted. Add the celery and onion and season with a small pinch of salt. Cook, stirring occasionally, until the celery is slightly softened, around 8 minutes. Add the garlic, parsley, rosemary, sage, and thyme. Cook, stirring, until the garlic is fragrant and the butter is melted, around 2 minutes. Taste for seasoning and adjust with salt and pepper as desired. Transfer the mixture to a bowl, add the toasted bread cubes and toss to combine.
In a separate bowl, combine the chicken bone broth and eggs. Whisk until combined and smooth. Little by little, pour the broth and egg mixture into the bowl with the sausage and bread. Toss to coat and let sit at least 10 minutes so that the bread can absorb the liquid. Depending on the type of bread you use and how dry it is, you may need more or less broth. If there is excess broth, add more bread. If the mixture is too dry, add more broth.
Grease a baking dish with 1 tablespoon of butter and pour in the stuffing mixture. Cover the stuffing with a sheet of parchment paper followed by a sheet of foil and bake for 45min at 350F. Remove the parchment paper and foil and bake until the top is golden brown, another 15 minutes.
Remove the stuffing from the oven and let it rest 15 minutes before serving.