2 tbsp vegetable oil
1 medium onion, finely chopped
1 stalk celery, finely chopped
1-1/2 Tbsp minced garlic
1 pound ground beef
1/2 pound ground pork
1/2 cup sour cream
1/2 cup Italian bread crumbs
1/4 cup Spicy mustard
2 tsp salt
1 tsp black pepper
1/3 cup chili sauce
Heat 1 tablespoon of the oil in a large ovenproof skillet over medium heat; add the onion, celery, and garlic and cook, stirring occasionally, until softened, about 5 minutes.
Transfer to a rimmed baking sheet, spreading out the mixture, and cool slightly.
Combine the beef, pork, sour cream, bread crumbs, 2 tablespoons of the mustard, the salt, and pepper; stir in the onion mixture.
Shape into 6 equal patties.
Heat the remaining tablespoon of oil in the skillet over medium heat and add the patties and cook for 7 minutes a side.
Transfer the burgers to a rimmed baking sheet.
Meanwhile, combine the chili sauce and the remaining 2 tablespoons mustard.
Preheat the broiler to high.
Spoon the chili sauce over each pattie to cover well and place the skillet under the broiler. Broil for 2 minutes or until the topping begins to bubble.