Cut the sirloin steak into 1/4-inch cubes, then season with the kosher salt and set aside.
Put the tomatillos into a saucepan with enough water to cover by an inch. Bring to a boil, then reduce to a simmer and cook for 10 minutes. Drain and add to a blender jar along with the garlic, onions, serranos, black pepper, and chicken stock. Blend until smooth.
Preheat your cast-iron dutch oven over medium heat and add the bacon. Cook until the bacon has released most of its fat and has become crispy. About 10 minutes. Remove to a paper-towel-lined plate and set aside. Pour off all but about 2 tablespoons of the bacon grease (I recommend pouring it into a heat-proof container and saving it for another recipe).
Increase the heat to medium-high, then add the cubed sirloin all at once. Cook undisturbed until the beef begins to release its juice, then cook, stirring constantly until the meat is browned. About 8 minutes.
Add the tomatillo puree and cilantro to your dutch oven, bring to a simmer, then reduce heat to low. Cover and cook for 20 minutes. Add the drained pinto beans, cover, and cook for an additional 10 minutes.
To serve, add to bowls, then top with the bacon and your fresh garnishes. Heat some flour tortillas in the microwave or oven and serve on the side.