Heat olive oil in a six-to-eight-quart pot over medium heat. Add bones and flanken; allow to brown. Transfer to a platter; set aside.

To the pot, add onions, leek, celery, parsnips, and carrots. Cook until onions begin to soften, about five minutes.

Return beef and bones to the pot; add split peas, salt, pepper, and eight cups water.

Bring to a boil, then lower to a simmer. Cook for one and a half hours, up to three hours for super-tender meat, adding more water if soup becomes too thick.

For puréed soup, remove the marrow bones and meat from the pot. Discard bones; shred the meat. Using an immersion blender, process soup until smooth; add shredded meat.

To store, refrigerate in an airtight container for up to five days. Soup will freeze well for up to three months.