Tear up the French bread into small bite-sized pieces to get 10 cups.
Spread the bread cubes on two 11x17-inch cookie sheets to air dry overnight, or toast them in a 400°F oven on separate racks for 10 minutes. If the bread pieces are larger, stir, rotate pans, and toast 5 to 10 more minutes or until golden brown.
Melt half butter in a large skillet over medium high heat. Cook the mushrooms for 8 minutes, stirring occasionally. Remove the mushrooms with a slotted spoon and place in a bowl.
Melt the rest of the butter in the same skillet over medium high heat and sauté the onion and celery for 7 minutes. Add the garlic and saute until fragrant, roughly 30 seconds.
Stir the spices into the onion mixture and cook for 1 minute to bloom the spices. Remove the onion mixture from the pan.
Turn the heat to medium-high and pour ½ cup of chicken stock into the skillet. Deglaze the pan by scraping the browned bits off of the bottom with a wooden turner. This will make the stuffing taste better. Pour back into the measuring cup.
Put the toasted bread cubes in a very large bowl and add the onion mixture and mushrooms. Mix together with your hands or with a large spoon if it's too hot.
Pour 2 cups of chicken stock into the mixture and use your hands to mix it until the bread cubes are moist, but not soggy. If needed, gradually add the last cup of stock to the stuffing. Taste and adjust seasonings to your liking.
Generously butter a 9x13-inch casserole dish, then spread the stuffing inside and pack it down.
Cook at 400°F for 30 minutes covered with aluminum foil, then remove the foil and bake for another 20 minutes. Serve this stuffing recipe while it's still warm.