Chop the onion and cook in oil. (add garlic if you have it and a Jalapeno or serano if you have it) Cook just till softened, don't brown. Add 1 tbs tomato paste and continue to saute for another minute or two.
Add rice to pan and stir for 1-2 minutes until well combined and add twice the amount of liquid as you did rice (if you added 1 cup rice, add 2 cups stock or water)
Bring to a simmer and cover and cook for about 15 minutes on very low
Add beef and combine and cover for another few minutes to warm the beef back up.