Ex-Boyfriend's Beef Stroganoff

Because it is the best beef stroganoff ever and because he finally gave me the recipe, so I don't want to forget it and neither should you. There are two key techniques with it, so look out for them.

A's Beef Stroganoff, serves 2-3
300-400g beef fillet*
1 brown onion
Bag of mushrooms
1 heaped ts tomato paste*
1/2-1 cup beef stock
1-2 ts Dijon mustard (Maille brand please)
Red wine (optional)
2 heaped Ts (lite) sour cream
Handful of chopped parsley

Thinly slice the onion and mushrooms. Heat a pan and in a little olive oil cook the beef as you would a steak. When cooked, remove and let the beef rest. In the same pan, add a little more olive oil and a nob of butter and saute the onions. Make sure they don't burn. Add the tomato paste and cook for a minute or two. Then add the mushrooms and continue to cook for a few minutes. Add the stock - if you want quite a lot of sauce add all the stock. When bubbling reduce the heat and simmer. Stir through the mustard. If you want a richer flavour add a goog glug of wine. Thinly slice the steak and add to the pan, along with any juices. Stir through the sour cream and finish with the parsley.
Serve with bowtie pasta.

* Do NOT use beef strips, unless you like the taste of cardboard. They are off-cuts of the worst meat. Use beef fillet, cook and slice instead, it's more expensive but juicy and tender.
* Whenever using tomato paste, always cook it off for a minute or two. It takes out that raw, 'seedy' taste and brings out the full flavour.