Heat a large pan with a little olive oil and seal the beef on both sides. Continue cooking in the oven to your liking. Meanwhile, using a large round cutter to cut cylinders of beetroot and potatoes then cut again into 1cm slices. Sauté in a pan using half butter and half olive oil until browned and cooked through. Next, heat a little olive oil in another pan and sauté the spinach leaves, season well with salt and pepper and remove off the heat. Once cooked allow the beef to rest for one minute then slice thinly. Serve on top of the spinach with the caramelized onion reduction and the potatoes and beetroot layered on the side.