Toast the Bread. Preheat oven to 350°F. Spread bread cubes on a large baking sheet and toast for 15-20 minutes until golden and dried out, tossing halfway through. Transfer to a large mixing bowl.

Cook the Mushrooms. Heat a large skillet over medium-high heat. Add 2 tablespoons butter and the olive oil. Once butter is melted and foaming, add the mushrooms in a single layer (work in batches if needed—don't crowd the pan). Let them cook undisturbed for 3-4 minutes until golden brown on one side. Stir and continue cooking another 3-4 minutes until deeply browned. Season with salt and pepper. Transfer to the bowl with the bread.

Sauté the Vegetables. In the same skillet, add remaining 2 tablespoons butter. Add onion and celery with a pinch of salt. Cook over medium heat for 8-10 minutes until softened and beginning to caramelize. Add garlic, sage, thyme, and rosemary. Cook 1-2 minutes until fragrant. Transfer to the bowl with bread and mushrooms.

Combine. Add parsley to the bread mixture. Pour warm stock over everything and toss gently. Let sit for 5 minutes so the bread absorbs the liquid. Add beaten eggs and mix until evenly distributed. The mixture should be moist but not soggy—add more stock if needed.

Bake. Increase oven to 375°F. Transfer stuffing to a buttered 9x13-inch baking dish. Dot the top with extra butter if desired. Cover with foil and bake for 25 minutes. Remove foil and bake another 15-20 minutes until the top is golden and crispy.

Serve. Let rest for 5 minutes before serving. Garnish with extra fresh herbs if desired.