Heat 4 tablespoons of the oil in a medium frying pan. Add the onions and garlic, cover and cook over a very low heat for 30 minutes, until very but not coloured.
Put the onion mixture in a bowl, add the mustard and mash it up. Stir in the parsley and vinegar, and add a dash of salt and pepper. Cover and put to one side.
Brush the steaks with a little of the rest of the oil. Press the crushed black peppercorns into both sides. Put the steaks under a medium hot grill and cook to taste: for 4-5 minutes on each side for medium-rare, or 5-6 minutes for medium.
Toast the bread on both sides. Spread 4 slices with the onion purée and cover with the slices of tomato. Add a piece of steak to each and top with rocket. Sprinkle with salt and pepper, cover with the rest of the toast and dig in while it’s hot.