In your largest skillet (with a tight-fitting lid) melt the butter over medium heat. Add the onion, garlic and mushrooms to the pan and cook for 2 to 3 minutes while stirring occasionally.

Meanwhile cut the steak into 1 1/2 inch chunks, season with salt and pepper, and coat them in the 1/4 cup flour.

Add the steak and any remaining flour to the pan. Feel free to add a little extra butter to the pan if it gets too dry. Evenly brown the meat on medium-high for 1 to 2 minutes then pour the stock into the pan.

Bring to a boil, while scrapping the brown bits off the bottom of the pan, then lower heat to a simmer, cover the pan, and cook for 5 to 10 minutes or until the meat is cooked all the way through and the sauce begins to thicken.

Remove the lid and add in 1/4 teaspoon salt, sour cream, mustard, and pepper (to taste). Simmer for another minute or two then serve warm over cooked whole-wheat egg noodles and garnish with fresh parsley if desired.