Preheat oven to 350 °F. Spread the bread cubes out on a baking sheet and dry them out in the oven for 15 minutes. Transfer bread cubes to a large mixing bowl.
Cook pancetta over medium heat until rendered about 10 minutes then transfer to the bowl with the bread. Add a drizzle or two of olive oil to the drippings along with the onions, celery, carrots and sliced garlic. Cook until soft about 10 minutes. Add the butter and melt. Mix in the chopped herbs and transfer everything to the bowl with the bread.