In a dish large enough to fit the beef lying flat, combine half the olive oil, balsamic vinegar, half the garlic, thyme and season with pepper. Add beef and toss to coat. Leave to marinate.
Heat the remaining oil in a large frypan in medium-high heat. Add the onions and garlic and cook for 6-8 minutes or until the onion is browned.
Meanwhile, cook the beans for 3-4 minutes in boiling salted water. Drain and rinse under cold water.
Transfer the onion mixture and beans to a plate.
Remove the beef from the marinade, shake off any excess into the dish. Cook for 3-4 minutes each side for meduim rare. Remove from the pan and place on a plate to rest.
Add beetroot to the pan, along with any marinating juices and cook for 2 minutes.
To assemble, place the rocket on plates, top with the onion and bean mixture and beetroot and pan juices. Thinly slice the beef and place on top. Crumble feta over the salad. If you would like more dressing, combine some olive oil and balsamic and drizzle over. Season with salt and pepper.
