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.
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.
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.
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.
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.