Preheat oven to 350°F, and lightly oil a 13” x 9” baking dish.
Spread the bread out in a even layer over 2 baking sheets. Bake for 10-15 minutes, or until bread is crusty but not hard. Transfer to very large bowl and cool.
While the bread is cooking, heat the oil in large skillet over medium-high heat. Add onions and celery. Sauté until they're tender, about 5 minutes. Add the sliced mushroom, and sauté until they are tender and beginning to brown, about 5 more minutes. Add all of the fresh herbs and the spinach and toss until just wilted, about 1 minute.
Stir vegetable mixture into the bowl with the bread cubes. Add the stock, salt and pepper, tossing to combine even. Add more stock, ¼ cup at a time if the mixture seems to dry. Transfer stuffing to prepared dish.
Bake stuffing uncovered until cooked through and brown and crusty on top, about 50 minutes. Let stand 10 minutes and serve.