Step 1

Position racks in the upper and lower thirds of the oven and preheat to 400 degrees F.

Step 2

Tear the boule into 1-inch pieces and evenly divide between 2 medium sheet trays. Drizzle with extra-virgin olive oil and sprinkle with salt and pepper. Toss to coat, then spread out into a flat, even layer.

Step 3

Bake on the upper and lower racks for 15 minutes; remove and toss, then switch the position of the sheet trays and bake until golden brown and crunchy, about 15 minutes. Remove from the oven and reduce the oven temperature to 350 degrees F.

Step 4

While the croutons bake, heat a medium nonstick skillet over medium. Drizzle in 2 tablespoons olive oil and add half of the mushrooms; spread into an even layer. Cook for 5 minutes, undisturbed. Stir and cook until golden brown, 2 to 3 minutes, then add the remaining mushrooms. Cook until golden brown and softened, about 10 minutes. Season with salt and pepper, then pour into a large bowl.

Step 5

Heat the skillet again over medium heat. Drizzle in 1 to 2 tablespoons olive oil and add the celery and scallions. Cook, stirring often, until they begin to soften, 8 to 10 minutes. Season with salt and pepper. Remove from the heat and add to the large bowl.

Step 6

Next, add the walnuts, thyme, rosemary, oregano and croutons; toss to combine. Add 2 cups of the broth, stir and let absorb for a few minutes, then add 1 cup. Stir and pour into a 9-by-13-inch baking dish; add the remaining 1/2 cup broth and finish with a drizzle of olive oil.

Step 7

Bake until golden and crispy on top, 30 to 35 minutes.