Mix all the sauce ingredients, except for the butter.
Cook the steak in a dry pan (no oil). Turn the cubes in the pan until they are golden brown on all sides.
Cooking the meat will produce a lot of oil, so after the cubes are cooked thoroughly, wipe excess oil with a paper towel.
Add vegetables and mushrooms. Stir in the remaining oil to coat, and add in the sauce ingredients from Step 1.
Cover the pan and steam over low heat for about 3 minutes. This will help reduce the acidity of the balsamic vinegar.
Remove the lid. While stirring, simmer the pan over medium heat until the sauce has reduced to about half the original amount.
Turn off the heat, and melt the butter into sauce in residual heat.
Garnish with parsley. Done.
Story Behind this Recipe
I like to use sugar and vinegar to create a rich flavour, so I make this often.
Use any brightly coloured vegetables. I didn't use them this time, but using I like to add in green beans to make the dish tastier and more colourful. If you want the dish to be less rich and more refreshing, omit the butter at the end of cooking.