Preheat oven to 350°F.
Cut the cornbread into bite-sized pieces. Place them on baking sheet and toast until they are dry and beginning to brown, about 15 minutes. Remove them from the oven and set aside to cool to room temperature.
While the cornbread is toasting, coat a large sauté pan with olive oil and place it over medium heat. Add the onion and cook until it's soft, about 7 minutes. Then add the celery and cook for another couple of minutes. Stir in the mushrooms and a bit more olive oil if necessary. Sauté the mushrooms until they are nicely browned, about 10 minutes. Turn the heat to low and deglaze the pan with the sherry, scraping any bits of onion, celery, and mushroom off the bottom of the pan and back into the mixture. Add the herbs and cook on medium heat until almost all of the liquid has evaporated. Season to taste with salt and pepper. (Here's How to Season to Taste.)
Add the toasted cornbread and the mushroom mixture in a large mixing bowl. Gently mix to combine. Add the desired amount of stock to the stuffing. (If it's too wet, it will become mushy. Be careful and add a little bit at a time.) Season to taste with salt and pepper.
Before serving, heat stuffing in covered dish just until it's heated through, about 20 minutes at 350°F. For added crispness to the top, uncover the dish for the last 5 minutes or so.