First of all we will prepare marinade material, for that take a bowl and mix brown sugar, soya sauce, vinegar, black pepper and salt.
Now take the beef fillet and dip it it marinade until it gets a fine coating of marinade.
Now pour oil in frying pan, and heat it well and then fry the beef fillet in it until it gets brown texture. Now remove the steak from pan and put it in a plate.
Now in the same oil fry the garlic and onions, wait until onions are light brown, now add black pepper, salt and chicken stock in it and cook it on low heat. After the gravy is set, add mushrooms in it and mix them all together.
Take another pan and put melted butter and flour in it. Now cook them until the mixture gets brown colour. Add mustard seeds and cooked flour in the mixture prepared in step 4 and then cook it for approx. 4 minutes.
When the sauce prepared in step 5 thickens, mix boiled potatoes with beef fillet and mix them with the sauce.
Place the cooked dish in a plate and to garnish spread thymes on it and then serve.