Heat butter in a skillet. Add the onion, carrot, celery, chile, and garlic. Sauté until vegetables are soft and the onion is golden (about 10 minutes). Let cool. Meanwhile, soak the breadcrumbs in milk until soft (10 minutes). In a large bowl put the pork, beef, egg, softened bread, cooled veggie mixture, salt, and pepper. Gently mix until just combined. Form into 6 patties. Grill until done. Add cheese, bacon bbq sauce, tomato, and lettuce over burger, if desired, or build burger to your liking.