In a stockpot, combine, water, beef and quartered onion. Bring to a boil, reduce heat to medium low, and simmer covered for 1 1/2 to 2 hours, or until meat is very tender. Strain and reserve broth. Shred meat with 2 forks; set aside.
Heat oil in a heavy skillet over medium high heat. Saute chopped onion and bell pepper until tender. Stir in 1 cup of the reserved broth, shredded beef, salsa, cilantro and jalapenos. Cook, stirring occasionally, for 4 to 5 minutes, or until heated through.