Mix the wine, soy sauce, mustard and sugar in a big bowl. Toss in the steak strips, season well with pepper and leave to sit and soak while you get everything else ready.
Pour the oil into a large frying pan and heat it up over a medium heat for a minute or so. Tip in the onion slices, turn the heat up to very high and cook for 3-4 minutes, stirring often to get the onions really golden brown. Take the onions out and keep them warm.
Lift the steak strips out of the marinade with a slotted spoon and stir fry them in the onion pan over a very high heat for 30 seconds (do it in two batches if it’s easier). Pour in the marinade from the bowl and boil for a few seconds until it sticks to the meat but is still juicy. Season with salt and pepper and keep warm.
Toast the bread and then spread with the garlic butter and cut each slice in half. Overlap two pieces of toast on each plate, top with a spoonful of onions, then the steak and more onions. Put a little pile of watercress on each plate and serve straightaway.