Thinly slice the onions. Add a knob of butter to a frying pan, once foaming add the onion, a pinch of salt and cook over a low heat until soft and golden brown (5-8 minutes)
Meanwhile combine your cream cheese and cheddar in a saucepan. Cook over a low heat until it becomes a thick and creamy cheese sauce.
Heat a separate frying pan until it's smoking.
Trim any fat off the steak and slice across the grain into very thin strips. Put in a bowl with lemon, oil and season just before it hits the pan. Cook for around 10 seconds per side or until brown. Allow to rest while you assemble.
Assemble- onions on the bottom, steak in the middle and cheese sauce spread on the top piece of bread.