Firstly, make the dressing by putting everything in a small jar with a tight fitting lid. Shake well until thoroughly mixed and season to taste with salt and freshly ground black pepper.
Heat the oil in a heavy based frying pan until smoking hot. Season the steak on both sides with a little salt and freshly ground black pepper. Lay the steak in the pan and fry, undisturbed, on one side for 2-3 minutes. Flip over and fry on the other side for a further 2-3 minutes. For a 2-3cm thick piece of meat these timings should give you a medium steak. Cook a little less for rare, a little more for well done. Remove the steak and allow to rest on a warm plate tightly covered in foil.
Add the shallots to the frying pan and fry for a few minutes over a medium high heat until softening and beginning to caramelise. Remove to a plate and keep warm.
Lay the ciabatta, cut side down, in the frying pan and toast lightly for a minute or two.
Slice the steak into 1cm wide strips across the grain and divide between 2 pieces of ciabatta. Top with the sliced beetroot and fried onions. Drizzle over a little of the dressing and cover with the top of the ciabatta. Any leftover dressing will keep in the fridge for at least a week.