Preheat oven to 200ºF. Arrange the diced bread on a baking sheet and transfer to the oven for 20 minutes. Meanwhile, grease a 9x13 baking dish with butter.
Remove the bread from the oven and transfer to the prepared baking dish. Turn the heat on the oven to 350ºF.
Cook the aromatics:
Place a skillet on the stovetop over medium heat. Add the butter. Once melted and bubbly, add the sliced leek. Cook, turning occasionally, for 6–8 minutes until it softens. Add a sprinkle of salt.
Add the celery and sauté for 5–6 minutes. Stir in the garlic, thyme, sage, and rosemary. Turn often for 2 minutes until fragrant. Season with salt and pepper. Turn off the heat.
Pour the sautéed aromatics over the bread and toss to combine.
Prepare the liquid:
Combine the chicken stock and eggs in a bowl and whisk until completely combined. Pour over the bread and use your hands or a rubber spatula to gently combine. Arrange into an even layer.
Bake the stuffing:
Transfer the baking dish to the oven for 45 minutes. Rotate it once midway through cooking to ensure it browns evenly. Once the stuffing has set in the middle and the top is golden brown, remove from the oven.
To serve:
Serve the stuffing warm. Keep leftovers in an airtight container for 3–4 days in the refrigerator or in the freezer for up to a month. Enjoy!