Soak the red onion in a bowl of water for approximately 15 minutes, then drain and set aside.
Lightly spray a large non-stick frypan with oil and place on medium to high heat. Season steak with salt and pepper. Grill for 3 minutes each side or until cooked to your liking. Remove from heat, cover and set aside to rest for 3 minutes before slicing thinly.
To make dressing, add all ingredients into a small bowl and mix to combine.
To serve, divide Baby Spinach leaves among serving plates and top with red onion and beef. Pour dressing over salad just before serving or serve in individual dishes on the side.