For the caponata, spray a large, wide non-stick pan with 3 sprays of oil, and add the onion and garlic. Cover and cook for 5 mins, stirring halfway through to brown them.
Tip in the tomatoes and a can of water, then stir in all the other caponata ingredients. Cover the pan and leave to simmer for 30 mins.
Heat a griddle or small non-stick frying pan. Generously grind black pepper over the steak and sear on both sides, about 6 mins in total, until cooked to your liking. Allow to rest while you wilt the spinach in a covered pan on a low heat.
Spoon the caponata onto 2 serving plates, top with the spinach, then slice the beef and arrange on top.