Combine the tomatillos, serrano peppers, garlic, and water in a small saucepan over medium-high heat; bring to a boil, cover, and simmer for 10 minutes. Remove the pan from the heat and allow to cool. Transfer the contents to a blender and blend until smooth. Set aside.
Cook the bacon in a large, deep skillet over medium-high heat until crispy, about 10 minutes. Drain on a paper towel-lined plate. Crumble the bacon and set aside.
Place a non-stick skillet over medium-high heat; cook the flank steak in the hot skillet until completely browned. Pour the tomatillo mixture over the beef and bring to a boil. Stir the chicken bouillon into the mixture, and reduce heat to medium. Cover the skillet and simmer until tender, at least 30 minutes and up to 1 hour.
Meanwhile, heat the pinto beans in a saucepan over medium heat until warm; reduce heat to low to keep warm until needed. Stir the bacon and pinto beans into the flank steak mixture; divide the mixture between 6 bowls. Garnish each with onion, cilantro, black pepper, and a lime wedge.