Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels. Crumble bacon and set aside.

Combine water, tomatillos, serrano peppers, and garlic in a small saucepan over medium-high heat; bring to a boil, cover, and simmer for 10 minutes. Remove from heat and allow to cool. Transfer contents to a blender; blend until smooth and set aside. (You can skip the simmering step and blend raw tomatillos, peppers, and garlic with water and a few pieces of browned flank steak if you like.)

Place a nonstick skillet over medium-high heat; cook and stir flank steak in the hot skillet until completely browned. Pour tomatillo mixture over beef and bring to a boil. Stir in chicken bouillon and reduce heat to medium. Cover the skillet and simmer until tender, 30 minutes to 1 hour.

Meanwhile, heat pinto beans in a saucepan over medium heat until warm; reduce heat to low to keep warm until needed.

Stir bacon and pinto beans into flank steak mixture; divide mixture between 6 bowls. Garnish each with onion, cilantro, black pepper, and a lime wedge.