Dust the beef with flour. Heat oil in a large saucepan and brown the beef on all sides. Remove and set aside.
To the saucepan, add the onion, garlic, carrots and celery (add more oil if necessary). Fry a few minutes, then add lentils and tomatoes plus juice and stir well.
Add beef stock, rosemary and seared beef. Simmer 40 minutes. Add potatoes and simmer another 20 minutes until done.