Step 1

Preheat the oven to 375 degrees F. Melt 2 tablespoons butter in a large skillet over medium heat. Add the onions and celery and cook until translucent, about 5 minutes. Transfer the contents to a bowl and set aside.

Step 2

Arrange the bread slices in a single layer on a baking sheet and toast in the oven, turning once, until light brown. While the toast is hot, lightly spread both sides with butter. Cut the toast into 1-inch squares and transfer to a large bowl. Add salt and pepper to taste, then add the thyme, sage and rosemary. Toss and set aside.

Step 3

Heat a large skillet over high heat; add the remaining 1 tablespoon butter, then add the pepperoni pieces and cook, stirring, 2 to 3 minutes. Drain on paper towels. Add the sausage to the skillet and cook, stirring, until browned, about 10 minutes.

Step 4

Add the celery and onion mixture, pepperoni, sausage and mozzarella to the bowl with the bread. Mix to combine, then add the chicken stock to moisten all of the ingredients; toss.

Step 5

Pour the mixture into a 3-quart baking dish and cover with foil. Bake 30 minutes, then remove the foil and continue baking until golden brown, about 20 more minutes.