Heat oil in a pot over medium-high heat. Add beef and cook until it releases its juices. Add the whole onion and let it simmer with the beef to release its flavor. Remove and discard the onion before adding the peppers.
Cover, lower heat, and simmer until the liquid evaporates and the meat begins to soften.
Stir in peppers and cook until softened. Add garlic and cook for 30 seconds. Stir in tomato sauce and cook for 1 minute. Add hot water and salt. Cover and simmer until the beef is tender and the sauce is glossy. Bon appétit.