Combine tomatillos, serrano peppers, onion, garlic, and 1 cup of beef broth in a blender. Blend until smooth.
In a large pot, cook bacon over medium heat until crispy. Remove bacon, chop, and set aside. In the same pot, add beef and cook until browned.
Pour the blended tomatillo sauce over the beef in the pot. Add remaining beef broth and bring to a boil. Reduce heat and simmer for about 45 minutes or until beef is tender.
Stir in cooked pinto beans and half of the chopped bacon. Simmer for an additional 10 minutes. Season with salt and pepper to taste.