Preparation: Finely slice the mushrooms and onions, mince the cloves of Garlic and slice the beef into thin strips.
Melt half the butter in a frypan over a medium heat and soften the onions. As the onions begin to caramelize, add in the minced garlic and cook for a further minute. If your pan is large enough, add in the remaining butter & mushrooms. Cook down the mushrooms until they start releasing their juices and they will caramelize further once the liquids have evaporated. (If your pan is not large enough, reserve the onion & garlic in a seperate bowl and continue to sautee the remaining butter & mushrooms as above)
Reserve all the cooked vegetables in a seperate bowl and start browning the steak strips in batches, setting aside the cooked steak pieces with your cooked vegetables as you work through browning the raw meat in batches.
Once all the steak strips are cooked, deglaze the pan with your dry Marsala. Continue by adding the thyme, mustard and chicken stock powder and gently stir to combine and release the brownings from the pan. Reduce this mixture slightly and then add the vegetables and steak back to the pan to warm through.
Remove from heat and stir through the sour cream. Serve on top of warm fluffy white rice, or with crusty bread. If you have any fresh parsley or similar green herbs, sprinkle some on top to finish off the dish.
(Note - the nutritional information provided assumes serving with 1/4 cup raw weight rice per person, cooked.)