For the sandwich, rub the steak with olive oil and season, to taste, with salt and freshly ground black pepper.
Heat a griddle pan until smoking hot, add the steak and cook for two minutes on each side. Add an extra minute cooking time if you prefer a well done steak.
Place a slice of ciabatta onto a serving plate, spread with tomato sauce, place the steak on top and arrange the spinach leaves on top of the steak.
For the dressing, place all the ingredients into a small blender and blend until smooth.
To serve, pour the dressing over the spinach leaves in the burger and top with the other slice of griddled ciabatta.