Heat the olive oil in a large pot over medium-high heat until it begins to smoke. Season the beef with salt and pepper, then gently place into the hot oil, and brown on all sides, about 8 minutes. Once browned, remove the beef and set aside, reserving the remaining oil in the pot. Stir in the celery, carrot, onion, garlic, cilantro, and oregano; cook and stir until the onion has softened and turned translucent, about 8 minutes more.
Pour in the chicken broth, tomatoes, and browned beef. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until the meat is beginning to turn tender, about 1 hour.
After the soup has simmered 1 hour, add the lentils, recover, and continue simmering until the lentils are tender, about 40 minutes. Season to taste with salt and pepper, then stir in the parsley.