Add the onions and cook for 10-12 minutes or until the onions are golden brown and caramelised. Add the mushrooms in the last five minutes. Set aside and keep warm.
Lightly coat both sides of the steak with cracked black pepper; heat extra oil in the pan; and pan fry the steak until it is cooked as you like it.
Remove the meat from the pan and keep it warm; add the garlic to the pan and sauté it for 1 minute; stir in the tomato paste, water and balsamic vinegar and simmer for 1-2 minutes.
Place the steak on a serving of mashed potato, top with the caramelised onions and mushrooms and drizzle with the tomato sauce.
Notes: I have not yet made this recipe, but when I do, since tomato paste disagrees with me (I find it too sharp), I shall be using a can of tomatoes, drained, and a teaspoon of a wonderful Australian spice Bush Tomato. If you can get hold of Bush Tomato, it's wonderfully aromatic and flavoursome without whatever is the ingredient in tomato paste that makes it sharp.