Pour flour into a shallow bowl and dust steak with flour. Heat oil in a large skillet over high heat and sear beef on all sides until browned, 3 to 5 minutes. Remove from the skillet and set aside.
Add carrots, onion, celery, and garlic to the skillet and cook until softened, about 5 minutes. Add more oil if necessary.
Add tomatoes with juice, lentils, beef broth, rosemary, and browned beef pieces to the skillet and stir well. Reduce heat, cover pot halfway with a lid, and simmer for 40 minutes.
Add potatoes to the skillet. Simmer until lentils and potatoes are soft, about 20 minutes. Season with salt and pepper.