Boil the salsa vegetables. Add the tomatillos, garlic clove, and serrano pepper to a pot of water. Boil until the tomatillos turn from bright green to olive green and soften, about 10 minutes.
Prep the beef. While the tomatillos cook, trim all visible fat from the beef and cut it into small, evenly sized cubes so everything cooks at the same rate.
Cook the potato and onion. Peel and dice the potato. In a skillet, add a small amount of water and a pinch of salt, then add the potato and diced onion. Cook until the potato starts to soften.
Blend the salsa. Transfer the tomatillos, garlic, and serrano to a blender with a splash of the cooking water. Blend until smooth and season with salt to taste.
Bring it together. Add the beef and the green salsa to the skillet with the potatoes and onion. Stir to combine.
Simmer. Cover and cook over low heat until the beef is tender and the salsa has thickened slightly, about 15 to 20 minutes. Serve hot.