Add oil to a nonstick skillet, and place over high heat. Add white and light green parts of sliced green onion; reserve the darker green part for garnish. Add beef, and use a spatula to break into small crumbles, about 5 minutes.

Reduce the heat to medium-high. Add salt, black pepper, garlic powder, chili powder, cumin, oregano, tomato paste, and ketchup and cook, stirring, for about 2 minutes.

Use a spatula to smooth and pat the mixture down into an even layer. Let cook without stirring for 1 to 2 minutes more, or until the pieces of meat start to form a brown crust. Give the mixture a stir and repeat the browning process once more.

Add in the water, and continue to cook, stirring occasionally, until the meat mixture reaches your desired texture, 1 to 2 more minutes—cook briefly for a saucier version, or longer for a dryer, stickier version. More liquid can always be added to the pan to adjust if needed.