Thinly slice the onion & garlic.
Cut off the fennel stalks, discard. Cut off the feathery fronds, put these aside. Slice or shred the fennel.
Heat the oil in a heavy-based frying pan. Add the onion, cook until translucent, then add the garlic, fennel seeds and fennel. Cover and sauté for 5 minutes.
Add the broth and cook for 10 minutes, until tender. Take off the heat and transfer to a dish.
Wipe the pan and heat it up again.
Oil the steak then season with salt. When the pan is hot, fry the steak for 30 seconds, then turn over. Fry again for 30 seconds and turn over. Keep doing this, for 4-10 minutes, depending on how well done you like beef.
Add half of the balsamic vinegar during the last 3 minutes of cooking.
Rest the steak on a plate under cover.
Drain the lentils.
Put the fennel mixture back to the pan, heat it up and add the lentils and the remaining balsamic vinegar. Add some more broth if it's sticky. Cook for about 5 minutes until the sauce thickened.
Slice the beef 1 cm thick and serve on top of the fennel & lentil stew.
Sprinkle with the fennel greens.