Cut roast into strips ½ inch thick and 2 inches long. Use a lean roast and remove all fat and gristle. Season strips with ½ teaspoon each salt and pepper.
As to the onion. When using green onions, discard the green. The flavor benefit is minimal and the color added is not attractive. You can substitute a ¼ of a medium vidalia (or sweet) onion if you prefer.
Use your largest skillet or chef's pan for this step. Melt butter in pan. Brown beef strips quickly over medium heat.
Push browned beef strips aside to edge of pan. Add onions to pan, cook 3-5 minutes. Push onion aside with beef strips.
Tip the pan slightly to collect juices. Stir in flour (you may wish to use a whisk for this). Mix until smooth. Add beef broth, stirring constantly and bring to boiling.
Lower heat to simmer and stir in mustard. Mix beef and onions back into the juice. Cover and simmer for 1 hour.
Before serving, add mushrooms, sour cream and white wine. Raise temperature briefly while stirring so they don't cool the dish too much. Add salt and pepper.
While this recipe calls for white wine, I, often with a surfeit of red and a dearth of white wine in my kitchen, have found that red wine works quite well. Regardless of color, avoid the driest wines. They will add an aftertaste to the meat that I find unpleasant.