Beat the steak with a rolling pin until it is about the thickness of two £1 coins, then cut into two equal pieces. In a bowl, mix the balsamic, thyme, grated garlic, half the oil and a grinding of black pepper. Place the steaks in the marinade and set aside.
Heat the remaining 1 tsp oil in a large non-stick frying pan, and fry the onions and garlic for 8-10 mins, stirring frequently, until soft and starting to brown. Meanwhile, steam the beans for 4-6 mins or until just tender.
Push the onion mixture to one side in the pan. Lift the steaks from the bowl, shake off any excess marinade, and sear in the pan for 2½-3 mins, turning once, until cooked but still a little pink inside. Pile the beans onto plates and place the steaks on top. Add the beetroot wedges, olives and remaining marinade to the pan and cook briefly to heat through, then spoon on top and around the steaks. Add the rocket and serve.