Place potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until almost tender but still firm, about 15 minutes. Drain and finely chop.

Cook beef in a large cast iron or nonstick skillet over medium heat until fat begins to render. Stir green onions, salt, and pepper into beef. Add garlic and butter; cook and stir until butter is melted and garlic is fragrant, 1 to 2 minutes.

Stir potatoes, salsa, and a pinch of salt into beef mixture. Pat mixture down in the skillet with a spatula and divide hash into 4 sections.

Cook, without stirring, until a crust forms on the bottom, about 5 minutes. Flip sections and cook until a crust forms on the bottom, about 5 minutes more.