Preheat a barbecue or chargrill pan to medium-high heat and bring steaks up to room temperature.

Brush the onion slices with 1 tablespoon of the olive oil and season with salt and pepper on both sides. Cook onions on the barbecue for about 3-4 minutes on each side or until the onions have caramelised slightly.

Transfer the onions to a medium saucepan, place over medium heat with 1 tablespoon of oil.

Brush the steaks with 1 tablespoon of oil and season with salt and pepper on both sides. Cook the steaks on the barbecue for about 3-4 minutes each side for medium rare, then remove from the barbecue and rest for 4 minutes, lightly covered with foil.

While the steaks are cooking, continue cooking the onions in the saucepan for 3-4 minutes or until light golden brown and tender. Add the balsamic vinegar and swirl to deglaze the pan.

Add the cream and chicken stock and simmer for 2 minutes.

Transfer the sauce to a blender and puree until smooth, being careful as the steam from the hot liquid can cause the blender to overflow.

Place a large frying pan over high heat and add the remaining tablespoon of oil. Once oil begins to smoke, add the spinach and toss for 30 seconds, then add the tomatoes and cook until spinach wilts and tomatoes are hot.

Season to taste with salt and pepper.

Using a large sharp knife cut each steak into thin slices and divide among four serving plates.

Transfer the spinach and tomatoes onto the plates, spoon the onion sauce over and around the steaks and drizzle with any remaining pan juices.