Heat the olive oil in a wok or large frying pan. Season the steak with salt and freshly ground black pepper and stir fry over a high heat for 2-3 minutes. Remove the steak from the pan and set aside in a warm place.
Add the fennel, onion, leeks and garlic to the pan and stir fry for 3-4 minutes.
Return the steak to the pan, add a splash of water and season, to taste, with salt and freshly ground black pepper. Stir fry briefly until the steak is cooked to your liking. Place into a serving bowl.
For the croutons, heat the oil in the wok and fry the bread pieces until golden-brown and crisp.
To serve, sprinkle the croutons over the stir-fry.