Brush a 9x13-inch glass baking dish with 1 Tbsp. olive oil. Set aside.
Heat 1 Tbsp. oil in large skillet over medium-high heat. Add celery and onions; sprinkle with salt and pepper. Sauté until vegetables are soft, about 5-7 minutes. Scrape into very large bowl.
Add 1 Tbsp. oil to same skillet. Add mushrooms; sprinkle with salt and pepper. Sauté until mushrooms brown and liquid evaporates, about 4-5 minutes. Scrape mushrooms into bowl with celery.
Add remaining 1 Tbsp. oil and sausage to same skillet. Sauté until brown and cooked through, about 8 minutes; add to bowl with vegetables.
Add the broth and brandy to the same skillet. Bring to simmer, scraping up browned bits. Pour broth mixture into same bowl. Add in the herbs, salt, and pepper and combine.
Mix in bread. Spoon stuffing into prepared dish. Cover with plastic wrap and refrigerate overnight or for at leas 6 hours.
Preheat oven to 375°F. Remove plastic wrap; cover stuffing with foil. Bake 30 minutes. Uncover; bake until top begins to brown, about 25 minutes longer. Or if you don't have time to wait 25 minutes, broil and watch carefully so tops don't brown.