Preheat the oven to 325 degrees F.
Toast the bread cubes on baking sheets for about 25 minutes, stirring once or twice, until lightly brown and crisp. Transfer them to a large bowl.
Butter a 9 by 13-inch baking dish and set aside.
Brush the leeks and mushrooms with oil and grill over medium heat until tender. Slice the white and pale green part of the leek and add them along with the mushrooms to the croutons. Warm 6 tablespoons of butter in a skillet and stir in the celery. Saute until soft, about 7 minutes. Add the sage, thyme, salt, and pepper. Stir to combine and scrape into the bowl of croutons. Add the stock 1 cup at a time until the bread is very moist but not soupy. Cover and refrigerate the dressing until ready to use.
Turn the oven up to 425 degrees F.
Taste and adjust the seasoning of the dressing. Whisk the eggs and baking powder together and incorporate into the dressing. Spoon the dressing into the prepared baking dish and cover. Transfer to the oven and bake for 25 minutes. Uncover and continue baking 15 to 20 additional minutes, or until lightly browned and crusty on top.
Garnish with the herb sprigs and serve the dressing hot.