Preparation Melt 1/4 cup butter with olive oil in heavy large skillet over medium-high heat. Add diced wild mushrooms and sprinkle lightly with salt and pepper. Sauté until mushrooms are tender and beginning to brown, about 8 minutes. Transfer mushrooms to large bowl. Melt remaining 1/2 cup butter in same skillet over medium heat. Add onions and celery. Sauté until vegetables are tender, about 12 minutes. Add all herbs; sauté 1 minute longer. Add spinach and toss until just wilted, about 1 minute. Add vegetable mixture to bowl with mushrooms. DO AHEAD: Can be made 1 day ahead. Cool, cover, and chill. Preheat oven to 350°F. Divide bread between 2 rimmed baking sheets. Bake until bread is crusty but not hard, reversing sheets after 5 minutes, 10 to 12 minutes total. Transfer to very large bowl and cool. Butter 13x9x2-inch baking dish. Stir vegetable mixture into bread. Whisk eggs, salt, and pepper in small bowl to blend well; whisk in 1 cup broth. Add egg mixture to stuffing, tossing to combine evenly and adding more broth by 1/4 cupfuls if dry. Transfer stuffing to prepared dish. Bake stuffing uncovered until cooked through and brown and crusty on top, 50 to 60 minutes. Let stand 10 minutes.
Preparation Melt 1/4 cup butter with olive oil in heavy large skillet over medium-high heat. Add diced wild mushrooms and sprinkle lightly with salt and pepper. Sauté until mushrooms are tender and beginning to brown, about 8 minutes. Transfer mushrooms to large bowl. Melt remaining 1/2 cup butter in same skillet over medium heat. Add onions and celery. Sauté until vegetables are tender, about 12 minutes. Add all herbs; sauté 1 minute longer. Add spinach and toss until just wilted, about 1 minute. Add vegetable mixture to bowl with mushrooms. DO AHEAD: Can be made 1 day ahead. Cool, cover, and chill. Preheat oven to 350°F. Divide bread between 2 rimmed baking sheets. Bake until bread is crusty but not hard, reversing sheets after 5 minutes, 10 to 12 minutes total. Transfer to very large bowl and cool. Butter 13x9x2-inch baking dish. Stir vegetable mixture into bread. Whisk eggs, salt, and pepper in small bowl to blend well; whisk in 1 cup broth. Add egg mixture to stuffing, tossing to combine evenly and adding more broth by 1/4 cupfuls if dry. Transfer stuffing to prepared dish. Bake stuffing uncovered until cooked through and brown and crusty on top, 50 to 60 minutes. Let stand 10 minutes.